i'm hoping it's 8 years before they introduce a new model, so i can be driving a current model until i get tired of it and trade it in for something else

 

Do you think the IS will ever get the L-shaped led headlights?

 

Sales
Some buyers will be turned off by the stagnating power output, however the main reason for the decline in 3IS sales is due to the increase in the number of competitors in this segment of the market as Cadillac has the ATS compact which sold 30,000 to 40,000 units per year, over the past two years.
http://www.goodcarbadcar.net/2011/01...s-figures.html

To make things worse, Jaguar is coming out with their compact XE too.

Furthermore, Benz has increased the sales of their C Class in recent years:
http://www.goodcarbadcar.net/2011/01...s-figures.html

It is not just the IS that has lost market share:
http://www.goodcarbadcar.net/2011/01...s-figures.html

The 3 Series benchmark at certain times, has lost some market share too, or it certainly hasn't gained market share unlike the C Class:
http://www.goodcarbadcar.net/2011/01...s-figures.html


Power
The reason why the 3IS and 4GS have become slower, is because over the past generation, cars have become bigger, and in particular, heavier; yet the engine has essentially stayed the same.
Lexus isn't pouring much more of its money into atmospheric engines.
Money is going into high aluminium content chassis and small capacity turbos now.

Benz is now ahead of Lexus in tech, because not only did Europe's expensive gasoline and small cities with strict emissions standards stimulated the Europeans to go small capacity turbo, they also stimulated the current C Class to have over 50% aluminium content.

Toyota/Lexus knows this, but their "safe" personality means they must take more time to engineer and refine their products before it is released onto the mass market, to maintain their durability and reliability over the current model C Class's very average reliability performance:
US:
http://www.jdpower.com/cars/search/2...ll-mpg-ranges/

http://usnews.rankingsandreviews.com...s/Reliability/

UK:
http://www.autoexpress.co.uk/Mercede...ss/reliability

Thus, Lexus presently refuses to assemble their cars in China:
http://www.motorauthority.com/news/1...-quality-fears

Meanwhile, the current C Class is assembled in Germany, South Africa, India, Beijing and Malaysia.
Benz's compact GLK CUV was previously assembled in Germany, Egypt, China, and even Ho Chi Minh City Vietnam.

Likewise, the 3 Series is assembled in: Germany, Mexico, South Africa, Egypt, Kaliningrad Russia, Chennai India, Shenyang China, and Jakarta Indonesia.
Meanwhile the X3 is assembled in: Carolina USA, Chennai India, Kaliningrad Russia, and Malaysia.

The Lexus IS and NX are exclusively assembled in Japan. Period.



Future
Theoretically, the next gen 5LS should have come in 2014, but if I'm not wrong, it was deliberately delayed till late 2016 as a 2017 model, to give extra development time for the next generation with high aluminium content bodies, and small capacity turbos.
Tech will trickle down the Lexus range.

The IS is based on the GS, such that the IS has so far been released roughly 12 months behind the GS.
Therefore, with the refresh/updated 4.5GS to be released next month, the refresh/updated 3.5IS should be released this time next year, with minor touches.

With a typical 7 year model cycle, the all new 5th generation 5GS should be released 2019.
Thus, the next gen 4IS with a high aluminium content body and small capacity turbo V6 should be released 2020.

 

The 2nd gen stayed around so long because the economy took a huge dump and Toyota needed to spend their R&D money on vehicles that sold in an appreciable volume.

 

Except for 1GS which wasn't born till 1993, so only had a for year model cycle, all GS's have so far had a 7 year model cycle to match Benz, BM & Audi.

The IS has always been released 1 year after the GS as technology trickles down the line.