Thurmond played really well this preseason, aside from game 1. According to PFF, he was even positive on run defense, on limited snaps, which was Winfield's main selling point.
These are the potential reasons I've heard of or thought of for letting Winfield go:
1. Lost it
Seems unlikely. Last year Winfield was top 5 in defensive stops according to PFF, in company with, e.g., Patrick Willis. He was similarly highly rated by Advanced NFL Stats. Physiologically, humans decay gradually. You would expect elite years to be followed by a couple "only good" years. Maybe he didn't follow his usual off-season program, but his circumstances seemed pretty motivating and he seemed pretty motivated.
2. Scheme fit
Maybe accumulated lost athleticism over the last few years didn't matter in Minnesota because their scheme let him use his good instincts and didn't expose him athletically. Perhaps in Seattle slot corners are in deep trouble if they don't have elite athleticism. The fact that we have a hard time getting off the field on 3rd downs and that Trufant looked so bad last year supports this theory.
3. Money
I think his base salary was only 500k? But he had pretty easy playing time incentives to get paid substantially more, I think. So either he was going to get paid quite a bit or he wasn't going to play much, either of which being grounds for being cut.
4. Defensive line injuries and suspensions
We are relatively strong at CB, but last year the defensive line was weak and after an off-season of trying to make it strong, we STILL have question marks. This must be frustrating for the staff and releasing Winfield makes it easier to keep someone like Brooks.
5. Releasing Mike Rob
Letting Mike Rob go and keeping Winfield might look bad in the locker room.
6. Culture fit
From what I've heard, Minnesota's training camp is much more relaxed than Seattle's. Old habits die hard. Maybe Winfield didn't fully buy in, especially since he's a future Hall of Famer. Maybe he didn't always compete.
7. Knee injury
The reason given for Winfield not playing the last couple preseason games was a knee thing. If that wasn't just a cover, that would make the staff nervous.
8. Youth principle
Seattle errs on the side of youth. Winfield didn't get much of a chance to show what he could do, but older players are on a shorter leash. Possible knee problems would further bring this principle into focus. In any case, it's a priority.