Interesting point. So I looked up from USAToday's archives and from the Jets' dude's salary cap page, and here's what I got. Here's the percentage of the Pats' Cap Value Brady took up:
Year - Total Cap Value - Brady's Cap Value - Brady's %
2001 - $54,404,881 - $314,993 - 0.58% - Pats win SB
2003 - $68,501,114 - $3,323,450 - 4.85% - Pats win SB
2004 - $70,383,779 - $5,062,950 - 7.19% - Pats win SB
2007 - $100,746,881 - $7,345,160 - 7.29% - Pats lose SB
2009 - $110,645,593 - $14,627,280 - 13.22% - Pats lose SB
2012 - $92,046,794 - $8,000,000 - 8.69% - Pats lose SB
I'm not responsible for the numbers, and I confess I'm not 100% sure I'm reading them right, so someone else can sure correct me if I'm wrong. But it certainly appears that Brady is consuming a much larger portion of the Pats' payroll than he was in the first few years of his career. And that should be self-evident. He was a 6th round draft pick and his first contract reflected that. As he became a star, his subsequent contracts reflected that development.
But the point here is that the more money that's spent on Brady, the less money there is to go around to the rest of the team. That also is self-evident. And that's all well and good so long as Brady produces accordingly (which he is). But if/when the decline comes, and if he's still taking up that percentage of the available cap space, then it becomes harder for the team to develop the rest of the complementary pieces to go with a declining Brady. And that also seems self-evident.
In fact, these points are so obviously true that it's very difficult for me to understand how anyone could be arguing about them.