I attribute it to too much spent studying the Madden NFL player numbers. If you study defenses, player characteristics making them appropriate for particular defenses, or just listen to player opinions of Seymour (other than one whining OT) this questions shouldn't pose much of a challenge.
Peppers is not a 3-4 DE, he is a 4-3 DE. This reminds me of last years grab Taylor discussion (as a DE, he is a light speed rusher who does not want to engage a lineman head-on as a 3-4 DE must or permit the offensive linemen to hit a linebacker and blow the defense up). I would love to see Taylor play OLB, but no way do I want to see Peppers take a stab at OLB at $16 million/year, or even half that amount. There is absolutely nothing to suggest he can put on 30 pounds and play a 3-4 DE role, nor do I see the Pats dumping Seymour, Wilfork and Warren to convert the defense to a 4-3 to adopt Peppers (certainly in light of the fact he doesn't want to play DE anymore).
Seymour is worth 5 Peppers in a 3-4. Short of Peppers taking $6 million to try OLB, he is not worth the cost, leaving Seymour entirely out of the discussion. No way would I let Seymour go to bring Peppers to the team.