In the five drafts from 2005-09, there were 18 QB's taken from picks 20-100. Of that group, only Aaron Rodgers is what I would consider a long term starter. If you add in the 2010 and 2011 drafts, there were 6 guys taken in this range, and early returns indicate that 1 of them will be a long term guy (Dalton) with one having potential (Kaepernick). That's 24 guys with, at most, 3 having a high enough ceiling to be a starter.
Each draft is different, but with the value teams place on QB, finding a guy late in the first or in the second and third isn't that great of odds. This is what happens when you don't get one from the beginning and stubbornly reject conventional wisdom of taking one high early in a rebuild.