MG - Let me preface this by saying I have been a huge Seymour fan. Seymour's jersey was the 2nd one that I got.. Brady's was the last one. Big Sey was an integral part of this team and he's got 3 rings millions of dollars to support that.
That being said, lets look at reality. Seymour hasn't played at an ALL-PRO level since the end of 2005. Yes, he seemed to rebound last year, but he still wasn't at an All-Pro level. So he's a former All-Pro.
BB did more than just trade so the "Pats can have a 2011 draft choice." The Pats took what would have been a 3rd round comp pick in 2011 and traded it and 1 year without Seymour for a 2001 1st round pick that is likely to be in the top 15, if not the top 10. That's 2-2.5 rounds of improvement in all likelihood. Not only that, but BB eliminated any possibility of there being distractions caused by any negotiations or lack there of on a contract extension for Seymour. We know that the last contract extension talks didn't go well and it led to Seymour holding out.. But we don't know what other distractions it caused in the lockerroom. And BB may have been pro-active in handling that. Particularly if they had any preliminary talks and Seymour's demands were outrageous.
Also, I think that BB looked at how the 4-3 Jumbo played and didn't like it. For whatever reason. Is he taking a risk??? Yes. How big of one we don't know. I mean, technically, going to the 4-3 as their base is a risk since they had built the team around the 3-4 for many many years.. So, is it much more risk?
Also, there was a lot of people who noted how impressed they were with the play of youngsters like Brace and Pryor. BB probably figured that he's better served giving them the reps at DT and someone else reps (Green/Wright/Ninkovich/Burgess/etc, etc) at DE than he was giving the reps to a player who was probably going to be gone at the end of the season.
In the end, BB clearly felt that the Pros outweight the Cons. Could he be wrong? Sure. He's human. But his history speaks for himself.