Well, I've never played football before, so forgive me in advance for speculating here..
But isn't the act of blocking much less physically punishing than the act of being tackled? Linebackers aren't looking to lay the wood to the fullback, they're looking to knock the piss out of the guy carrying the ball. Right?
Furthermore, I would think that the position of fullback requires much less athleticism, or "jukeability" (or even "the iso-motion") if you will, so if a running back's knees were to start failing him, the loss in speed and ability to cut would not hurt his ability to play as a fullback.
If those are the case, then I can see Bettis' career being prolonged through next year or maybe even two, although I don't see why he'd want to do that.