Hello docspencer
As others have said the supposed short lifespan of Plasma Displays is an urban myth - along with annual re-gassing of the pixel array!!!
Half brightness on a modern sixth/seventh or eighth generation Display (depending on which manufacturer your looking at) is usually brighter than full brightness on a 'new' Display back in the early days of first and second generation displays.
The display doesn't stop working at 60,000 hours it simply produces less light output - though still more than enough to view a 'TV' picture; that's assuming you still have the Display in 20 years time.
In the longer term the cost of replacing a major component once the Display is out-with the manufacturers warranty period is much more of a worry for most - though you can take out extended warranties to minimise the associated risk/cost.
Joe
PS If you ever had the misfortune of visiting folks houses who hang on to HiFi and TV gear until its well past its sell by date you'd be amazed at what some folk sit in front of on a nightly basis viewing 1970/80's CRT TVs that can only just produce a colour picture.