So many QBs who didn't break through and win it all in their first 4 or 5 years went on to put it together and get it done after their 4th or 5th year in the league. For every Troy Aikman or Tom Brady who wins it out of the gates earlier in their career, there are tons who didn't win their first until after their 4th or 5th year in the league. Young, Favre, Elway, Peyton, Brees, Dilfer, Johnson, Foles, Rodgers.
And before you try and turn around and say you weren't talking about winning Super Bowls, that you were just talking about players improving in general, you might want to actually go dig into the stats of some really good quarterbacks.
In Peyton Manning's first four years, 1998 through 2001, he posted passer ratings of 71, 90, 94, and 84. His average passer rating in his first four years was about 85, with a ceiling in the mid 90's and a floor in the 70's. From 2003 through 2014, he averaged a passer rating of 103 with a ceiling of 121 and a floor of 91. He improved noticeably after his first four years, don't you think?
In Tom Brady's first four years, 2000 through 2003, his passer rating never climbed out of the mid-80's. You probably don't even need me to recite the stats to tell you how much he improved from there, right?
Same with Drew Brees.
Same with tons of guys.
Your premise that a QB can't improve on his first 4 or 5 years is so wrong it's bananas. It's one of the positions where guys not remaining in the physical prime of their early- and mid-20's is besides the point because so much of a quarterback's growth and development and success is about what's going on between their ears.