Really depends on if we are talking 80's LCD screens (of which MANY still work) or more modern ones. The technology has improved, but the basics have stayed the same. Some (including me) would argue that things used to be just built better as well
The pixels themselves are driven by a solid state transistor, so no real lifespan issues there.
The fluid in the panel is fine for over 100,000 hours below 150° F and above freezing.
The back lights are now rated for 30 to 60,000 hours... and that is the half brightness rating, the thing that kills florescent lamps is powering them on when cold, the firing uses a small bit of the mercury to light them... eventually this is all used up.
So back lights aside (8 hours a day for 15 years is 45K hours).... the panel itself at the same 8 hours a day is rated for over 34 years.
There are many almost 20 year old (1992) ThinkPads still running, I would not expect the next 15 to be any different. Most electronics that are going to die do so in the 1st hours/days/months of their life.
If not left in a freezing/overheating situation (like left in a car, hot or cold), and handled nicely... the 1st thing to go is normally the inverter, then the CCFL, rarely does the panel itself die from a "just because" kind of thing.
I could be way off on a lot of that, I'm sure someone would chime in if so, but it's just my crude understanding of it (OR ... Just my 2 cents
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