The Seymour comparison is flawed. Seymour was just shy of 30 when he was traded; Mankins is 32 and has been injured more often. By 33, Seymour was pretty much an expensive JAG. That could be Mankins by next year.
Mankins has two years left at most. He is still a productive player, but age and injuries have taken their usual toll. There will be an immediate, negative effect on the offensive line. Makins made the others better. For example, with Mankins on his right, Solder could count on inside help so he could cheat outside just a bit on five step drop pass protection. That helped to cover Solder's biggest problem. Well, Mankins isn't there anymore. Bedard's passblocking criticism is unfair; Mankins covered sometimes for Connelly, Wendell and Solder on schemes; when you see him "beat" on film - at least some of those time it probably wasn't his assignment (I count twice in the Steelers game, for example). He was also the best run blocker, though that can be replicated a bit more easily in a zone blocking system; it looks like NE may be using man and zone now.