He's 36, but at 30, McNabb isn't a spring chicken anymore. Garcia hasn't been injured like McNabb either. Blow out your knee and that's bound to shave some years off your NFL life span.
I agree with your first paragraph (but I think the Eagles are closer to "rebuilding" than you apparently), but my point was this, if McNabb feels slighted, it wouldn't hurt to put out some interest. Especially from a superbowl ready team. Demanding a trade and all that. After all, McNabbs window for a superbowl is closing.