I was chatting to a Phase One dealer recently and mentioned this point.
He's proven himself rigorously forthright (he advised me against the Phase One/Hartblei T&S lens, saying every one he'd sold had subsequently been returned, and he'll cheerfully tell you why you should get a second hand Mamiya 120mm macro rather than the more expensive Phase One version), but his view was that having been involved in MF digital since the start he couldn't remember ever having a customer who's MF sensor had failed through use.
That's not to say backs didn't fail or need repairing, almost always because of accidents or mishandling, but his view was that the market doesn't realise just how long MF back life expectancy could be. For him the bigger issue was the advance of companion technology (ie connection cables or memory cards or RAW developer updates) rendering a back unsupported and redundant.
It's a question that's particularly interesting given Canon's recent announcement regarding the 1Ds MkIII replacement. Everyone was expecting yet more pixels, but it seems the 35mm sensor battle is capping at. No one knows what the future holds, but if that trends persists then second hand MF backs, in particular the 39 mega pixel or larger backs, could be in strong demand for many many years to come, and questions about their long term reliability will become even more frequently asked.