The problem is the Salary and what it does, or more to the point, prevents you from doing. Brady and the Pats have been able to bring in help and they have been able to keep key personnel because of their cap flexibility. And also, let me ask you this, why just 5 years? Why not 10? Is it because in the last 10 years, there have been 7 different winners and your "theory" falls completely apart? I think we both know that the answer to that is probably yes, that is exactly why you limit it to only 5 seasons.
But hey, you asked the question so lets go ahead and look at it. So cap flexibility doesn't effect anything? OK, who were the two teams?
It's Philly in 2018, who had a QB situation that paid their starter on a rookie deal and the eventual SB winning backup on a contract deal that paid him 8.7 Mil in base salary with incentives that could pay up to 31.9 mil with incentives over two seasons. In the SB winning year, I believe that Foles actually made 7 million. That allowed Philly to make several key FA acquisitions. and this is a known fact.
The other was Denver who had Manning at the time. Manning's base salary was 15 million in that year and Denver was able to have cap flexibility to sign
DWare, Talib, TJ Ward, Darian Stewart, Evan Mathis, Owen Daniels and Emmanual Sanders, to say nothing of Manning himself. All of those guys were FA signings who were starters on that team.
If anything, this only goes to show just how important it is to sign your QB to good and financially sound contract. I mean, virtually ever single team who has won the Super Bowl in the last 5 years has done so because they were able to take advantage of reasonable QB contracts.
I mean, this is plain as day. It's like a blast door that is closed, right in front of you and yet, you still walk head long it.