Teams can get better or worse at any point of the season. There's no point in saving better teams for later because for all we know Peyton might blow his knee next week and the Broncos could be 5-11 by the time the season ends.
Injuries do happen, but the greatest teams don't relie on just one person other then starting QB. But the QB's are insanely protected by the official rules to the point that rarely happen. Either they throw it away or protect the ball and themselves and pretty much sit down before getting tackled.
There's like 3 notable people out on the 49ers roster, but their still winning. "But Fire2box, they only won half their games." yeah, teams have slumps.