And people are looking at the past too much. Most of this contract is what can he do in the future? He is entering his prime. His level of in-game intelligence will only increase. Brady and Manning and Brees weren't picking apart opposing defenses in the first 5 years of their career, such skill comes later on. This is where Joe should be heading as well. We all know he has the physical tools. He should only get better and better at reading and diagnosing opposing defenses.
I keep seeing the "inconsistency" argument too for why he doesn't deserve top $$, but no one mentioning he had Cam Cameron his entire career. We've seen what happens when Cam leaves a QB - they excel. The last part of this season Joe pretty much dominated the competition. If I were his agent, I would say "Did you see what this guy did once Cam left? The future is bright". And I would use that as why he is worth so and so $$ in the future with respect to this new contract.
(And I for one HAVE made the argument that the inconsistency was because of Cameron, not Joe And that Joe accomplished what he did DESPITE Cameron, and a definite lack of true elite talent around him.)