If you make dead ball plays review-able, it opens up a HUGE can of worms, as has already been stated here. At that point, the refs whistle becomes meaningless. Players will truck on, blindside players, annihilate quarterbacks and kickers, etc.
Noone is arguing that this situation doesn't suck (except for Denver fans). It most definitely sucks. Ideally, SD would have shut down the candy plays that Shanahan thrown them, and made things right. They didn't.