No you can't bank on a new QB. But if you see an issue, you need to address it. Not let it rot and stink away. (Tony Romo is a great example of a franchise scared they have no other alternative so they paid him)
Paying someone just because you fear the mediocre QB can get worse if you choose to let him go to FA is a sure way to lose your chance to a SB.
But the way you win the other SB contender is by always having better players than the opponent. "Right player, right price" I always believe in that mantra. If you cannot do this when you pay FAs, you cannot win SBs.