Some people are mechanics, and some just wear that tag, without actually being one...
Recently came across the video on YT where a “mechanic” was stating that ATF WS should not be used in toyota cars, because it damages their auto transmissions. Sometimes it is quite obvious whether a person is a mechanic or not, lol

I guess you'd have to question what is your expectation of the amount of moisture that gets in.
The reality is that flushing the brake fluid system completely doesn't take that long, and presumably you're using new brake fluid.
There would be no more chance of moisture getting into the system than if you left the vehicle parked for the same amount of time.
When you flush the system you don't empty then fill it...it's a positive exchange where you continuously top up the reservoir as you bleed the system at each wheel.
This keeps air and moisture out the system. And for sure the pedal is going to feel a lot firmer after the flush.
Maybe there is some misunderstanding as to how fast brake fluid absorbs moisture...it's not gonna absorb anything that you can measure in an hour...we're talking about changing fluid every 2 years as a preventative maintenance...so you could let it go longer.
And the environment the vehicle is in will play a factor.
As an example, if you have two identical IS's, one in Ontario, Canada, and one in Miami, Florida, you can bet money that the battery in the Canadian vehicle will last almost twice as long...and it's the same battery.
Has nothing to do with the battery, it's the environment that kills the Florida battery faster.

All the mechanics I talked to said it will take around 2 hours to flush the brake lines during which
the brake fluid reservoir will be wide open.
I am pretty much convinced the only route that moisture can get into the brake fluid is thru the vent
hole on that brake reservoir cover.
How the moisture diffuses from the reservoir down to reach the brake lines is open for debate.
I wish there are people who can join the discussion if they have experienced brake failures due to bad brake
fluid.
So far, all the friends I talked to, no one has ever replaced brake fluid, and all their cars lasted around 200K miles
over 15-20 years.

First...if they are taking 2 hours to flush brake fluid, I'm sorry, but they have no clue what they are doing. If they want to charge you 2 hrs and it takes them 30 minutes, that's different...then they're just ripping you off. 1 hour labor plus fluid is more than fair...the tech has to make money too.
With a floor jack and 2 axle stands I could do brake fluid on a 3IS in an hour, with another person helping me pump the pedal...if I had 4 axle stands I could shave off another 15 minutes.
Secondly...the reservoir being wide open? the opening of the reservoir is about 1.5 inches in diameter...unless they are pouring water into the reservoir, where is this mysterious moisture coming from?!?!?!?!?! OMG!!! Unless you're doing this outside in the middle of a rain storm, it's not gonna get anything into the reservoir that is not deliberately put there. At the end of the day, the cap has a vent, so whether it's wide open when the cap is off, or somewhat open when the cap is on...it's not a sealed system.
And lastly, no one is likely going to experience a brake failure directly related to bad brake fluid...contaminated brake fluid can lead to corrosion inside the line, which could eventually lead to a leak, which could lead to the car not stopping...and that would then be a brake failure.
As the brake fluid gets more contaminated, the boiling point lowers more and more...it may not cause a brake failure, but it could make the difference when the car in front you slams on the brakes...and you stop before hitting them...or when you hit them.
If you've ever driven a car on a track and experienced brake fade cause the brake fluid boiled over, it's not a good feeling when you can't stop in the distance you expect the car to stop...or need it to stop.

I would suggest consulting a different mechanic that you've never dealt with before...preferably someone properly trained and qualified as an automotive technician.
Perhaps they can provide you accurate information, even if you decide not to flush the fluid.

As a general rule, people seem to more easily accept the information the want to hear, more than the information they need to hear...including myself.

For argument sake, the reservoir cap size as you said is 1.5 inches. The tiny vent hole is no bigger than 1/32 of an inch.
You know what is the area ratio. The reservoir cap when open is 2400 times bigger than the vent hole area. That is the
path moisture can enter and contaminate the brake fluid.

Sasnuke
You talked a lot about how moisture can damage the brake system. But moisture can only enter the brake system
thru the vent hole of the brake reservoir, Give me 1 reference that moisture inside the reservoir can diffuse into the
brake lines. Otherwise all your talks are just hearsays not science based.

Water from air will contaminate brake fluid via air to fluid interface. The speed of that process is determined by substances in contact, temperature and the area of that interface. If there is enough moisture to get absorbed via that interface, a bigger hole for the air will not accelerate that absorption.

Also, obviously, contamination is a function of time, so exposing brake fluid to air while flushing it is not the same as letting it run in a car for 2-3 years.
I mean, Lexus does recommend changing that fluid 2-3 years OR if the brake fluid test fails either the appearance test or water content test (whichever comes first). No sane manufacture would approve keeping the same brake fluid in a car for 5, 7 or even more years without changing it. That is insane. If you do, you will eventually boil that 5-10% water by volume brake fluid at extremely low t, and will end up losing brakes altogether.

Sudnamer: Thanks for your comments. I am facing the facts 1. We used the cars in my family
up to around 180K miles over 15 years and never replaced brake fluid. The friends we asked
they did the same thing. 2 Mechanics I talked to told me they never replaced brake fluid in their
cars, and they kept the car to around 200K miles. That led me to believe replacing the fluid may be
a hoax.
I am replacing the fluid tomorrow just because Lexus says so. But my Lexus Rx350 is already 9 years old
at 62K miles.

It is possible that you are so gentle with the brakes, and with the car in general, that you stay far away from boiling the fluid or getting anywhere near that. It is also possible that you do not drive your car anywhere close to its limit and/or use less aggressive pads in your car, so your brake pads (and fluid last longer)
Let me ask you -
1. What tires are installed on your RX350?
2. How many miles it took the front pads to get worn out?
3. What front brake pads do you have?

Rx350 has 69k in 9 years meaning 6888 miles a year on average. That is a half of what an average american driver covers in a year (13k miles).

I guess the first thing you have to understand is how brakes work...the reservoir is just a holder for the fluid...that's all it does.
Below the reservoir is the master cylinder that contain the valves that actually distribute the fluid to the front and rear brakes.
That process of distributing the fluid is how the fluid from the reservoir mixes with the fluid in the lines.
Here is a 4 minute video showing you how master cylinders work: And to be absolutely clear, everything I've stated in this thread is 100% fact, not fiction. But I realize your "mechanics" will tell you different...and that's what you want to hear.
If you think changing brake fluid is a hoax, then don't change yours...seems pretty simple.

Sunamer:
I don't drive much because I am retired. Still my son drives quite aggressively with his 1999 Honda
Accord with 160K miles on it. That is 20 years and no brake fluid change.

Sasnuke: Thanks for all the info. Still wonder whether the brake fluid level in the reservoir changes much
during operation. Might do an experiment tomorrow. My guess is the level will not vary much, so that
is not much of mixing of the fluid going on.

Sunamer, and my son-in-law is driving his 2004 Honda accord with 130K miles in LA, which has the worst
traffic in US interms of stop and go. Still original brake fluid. I might change his brake fluid when visiting him next time in LA.

An online quote:
I flushed the fluid on my girlfriend's 2000 Audi with 210,000 miles. The improvement in braking and brake pedal feel was amazing. So much so she actually noticed. She isn't too good at noticing stuff like that.
I don't get the disposal issue. Most auto part stores take the used fluid along with oil in the same waste oil container.

No the level does not change much...likely you won't notice it with the naked eye.
As the system is devoid of air, when the valves move in the master it only allows a very small amount of fluid to pass...just enough to put pressure on the caliper pistons to push on the pads to stop the vehicle.
Once you release the brake pedal, the caliper pistons naturally relax and push the fluid back slight.
So basically it's this seesaw action of the brake fluid between the master and the pistons.
This is why the recommended interval for flushing the fluid is years, not months...It will take a long time before all the fluid in the reservoir mixes with the fluid in the rest of the system.

Here is a link to a local Honda dealer explaining brake fluid replacement: https://www.richmondhillhonda.com/20...d-replacement/
Here's another video of brake fluid being changed on a Subuaru:
If you son likes to drive his Honda aggressively and has never changed the brake fluid, I can guarantee you that if he flushes it now, he will notice a difference in the braking immediately.