I think they'd already decided that Seymour was likely the one they would let walk after the season regardless; ergo, getting a 1st rounder is pretty compelling, especially given the depth at the position.
I've also been of the opinion that Seymour has been somewhat overrated over the years; he'll be absolutely dominant at times, at other times he'll be invisible. Some of that is scheme, but the defensive line as a whole has been inconsistent enough where I'm not sure anyone was really untouchable. Lots of talent, not always lots of execution, for the line as a whole and (at least as it appears to me) for Seymour specifically.