The more teams in the league the weaker the overall QB play is, there just aren't that many great QB's in the world at the same time.
Right now we have...
It's looking like Newton will be a great one someday.
Sure you can win without that elite QB (Niners, Ravens) but you need so many other things working for you. Far easier to win with that great Qb.
The top 10 retired
I think Bradshaw/Starr just had great teams around them.