he isn't quick to react. he just seems to be lead off by qbs too easily and cannot recover. he may be fast to get there but he is too far away to make all that speed worth anything.
he is marquise goodwin or tj graham for db's.
that's a snapshot of it because i'm tired but, yeah. he's just slow to move in football terms to get where he needs to be and slow to recover from his mistakes.
I feel like you're dramatically overstating your perception of Seymour. I just don't think he had enough opportunities on the first unit defense, and I don't think there is enough of a body of regular season work to make such absolute pronouncements-- especially considering the buzz he received in the preseason regarding his athleticism, and the potential that he showed. I also don't think you can say that he is only useful as a nickel, as that was the only position he was used for outside of special teams last season.
Assuming we draft a CB (safe assumption?), I believe that Seymour will be on equal footing in competition to start opposite Darby, unless maybe we end up with Lattimore. Even then, I wouldn't count him out. I think Seymour is better than you are giving him credit. But, then again, I am usually wrong about such things...