I know LCD displays "wear out" losing contrast over a many year period. We just discussed plasma, what I don't know is how DLP sets wear out. A web search on LCD life claims near infinite panel life. This has not been my experience, my old black and white LCD has noticeable loss of contrast in one year. Of course a number of different LCD technologies exist and it isn't safe to generalize too much.

I did a search and modern plasma sets deteriorate to 1/2 brightness in about 60,000 hours. If it were linear (unlikely), that would mean 10% loss of brightness in 4 years with 4 hours of TV per day. More likely for brightness is a faster fade when new, with the effect slowing down with time.

On DLP life, data is hard to find, everyone is focused on the life of the older projector bulbs or color wheels which was not very good. Reportedly TI has had a movable mirror IC (The heart of a DLP process) running since it's invention with no broken mirror hinges out of a million mirrors to date. LEDs also lose some light output with time, but it is very little and the colors do not change at all with age.

So all else being equal, the DLP set should maintain a bright picture longer than the other technologies. I do not know the replacement cost for any of the three LED (Red, Green and Blue) that power the light beam on a DLP set, they might be sold only in sets of three. Or they may not be available at all forcing a replacement of the whole TV if one fails.

I haven't ever seen a post of DLP LED failure, it's probably safe to assume it's a rare occurance.