I don't know about plasma TVs. That could be the source of this but LCD manufacturers are more confident now to quote lifespan numbers for the LCD panel to 60,000 hrs. Assuming 10 hrs per day usage that's more than 16years life. Of course the other parts of the LCDTV have similar lifespans to your old CRT TV and depending upon the brand, they should last as long.
I assume I would be able to enjoy my LCDTV for around 5 years and I don't limit its usage. By that time, much better models should be available. Maybe it should have a digital HDTV tuner by that time.