You were spot on with all the comments you made except one above 2 points. First you state that Seymour's has cost the Pats 2 seasons. That's just wrong. Seymour wasn't coming back to the Pats after the 2009. It was common knowledge that for whatever reason, Richard wanted a change of scenery. If you don't believe that, then its fair to speculate that the market price for Richard Seymour wasn't within the budget the Pats were dealing with, having to sign Brady and Wilfolk as well.
Secondly you imply that there is some flaw in BB management by NOT have an adequate replacement. That just plain unfair Richard Seymour was the best DLman in the league for most of his time in NE. He's still a top DLman in the league, as well as the highest paid one. He was also BB's ONLY pick in single digits....and he didn't miss. The point is that you can't simply replace him "adequately" Just about anyone you brought in would be a downgrade.
There is no question they missed him in 2009, but that was a flawed team to begin with. It was a good team, but the loss of Seymour wasn't the reason they didn't go farther than they did.
In the end I think it would be fair to postulate that the Pats would have been a better team with Seymour playing in 2009. But its overkill to think that it somehow would have been possible to sign him beyond that period.
Its also fair to think that with the resources that WEREN'T expended for Seymour were well used, including Nat Solder