Source?
Just means that one of two things will happen before you processor conks out from overheating and being all OCD about thermal paste is only a very small part of the longevity equation. other things are more likely to happen well before the processor goes south on you, namely:
1) you sell your machine (happens a ton based on the Mac fanboys on here and the turnaround of machines)
2) other stuff in the machine will burn itself out before the processor goes bad, ie. logic board, display, act of God instances...blah blah blah
there is logic to the statement and i wouldn't require a whitepaper writeup on it to prove a point. way too much effort.