I have another car in with a OE belt, this one is an 06 GS430 and here is how the belt is looking. The customer left it with me out of fear of driving it even a few more miles lol!

 

That is impressive! Which state this car is from?

 

Holy Cracks Batman!!!
He lucked out on that!
Looks like it's been somewhere HOT for most of it's life.

 

I mean... it's working, right? Doesn't matter if it's been somewhere hot or not... the engine gets hot, the timing set gets hot. The belt has 187K on it, apparently. Send that to Mistuboshi with a thank you letter for their quality.

 

Indeed it was working, on the test drive showing me issues he floored it to 100 and it didn't die lol!

 

Rubber outgases over time. More time spent at higher temperatures dries out the rubber and it cracks when flexed.
It has nothing to do with how hot the engine bay gets since that belt is always gonna be hot in use from the violent flexing it gets whenever the engine is running, even if driven in cold climates.
It's time spent sitting parked in hot dry air that frags rubber and your car spends a LOT more time parked than being driven.

As for the 187k miles & 16 years, that's pushing the recommended 90k/10 year interval somewhat. <LOL>

 

Yep, and rubber anywhere on the car will live longer with a little more humidity.