Sometimes players don't execute. Sometimes the other team executes better. Sometimes the right scheme against an opponent isn't what you are best at. No question Seattles offensive style and makeup are the kind that the Patriots tend to have trouble against.
I'm not sure why the sky is falling, they aren't getting it done, mentality is dominating the board after one poor game. Prior to this game the Patriots were allowing 16.5 ppg, which would now lead the league.
If you think 16.5 ppg was terrible defense then every team in the NFL has a terrible defense, and I would rather have the terrible one that lets fewer points on the board.
Ultimately, what we have here is a coach who has seen things on the way to that 16.5 ppg that told him he can do better. This is why there is a revolving door at CB2 and CB3 and at LB2 and LB3, as well as across the DL. This is why Collins is gone.
Belichick knows how to win. Win games, win divisions, win playoff games, win Championships. What he is doing with the defense is his belief on what will help win a championship, and no doubt this is a work in progress that no doubt will be better when it has to be. One bump in the road bad game is not something to worry about and part of the reason BB wins like he does is that he is not afraid to make the moves that will get him where he needs to be, even if it is a step back at first.
I will guarantee that by January the reps at DL LB and CB will not be the same as they were vs Seattle. The defense will evolve over the season. Personally, I think the proof is resoundingly clear that if BB says this is what I need to do to get my D where it needs to be, he is right. Does anyone want to claim they can challenge his track record?