Dilfer only has one ring, but I understand the gist of your argument.
Alex Smith is decent against bad defenses (see Saints), but likely will never do **** against above average defenses in playoff games. They still came very close to making a trip to the Superbowl and you can never say never, but I can't see Alex Smith ever being the starting QB for a Superbowl champion.
When your defense forces another team (especially one with a good offense like the Giants) to punt 12 times you have got to find a way to make a couple plays to win the game. Alex Smith more times than not will fail to make those plays against decent defensive teams.