As to Bill's longevity, the biggest factor in him staying is how much he enjoys doing the job. Clearly at this point, he does. As a fan who regularly goes to the TC practices over the years, you can see how actively he is involved in directly coaching players, from the starters to the camp fodder on both the offensive and defensive sides. He LOVES it. It's just not work to him. So as long as it feels that way to him, he will be here.
Brady is the GOAT. To think the team will be as good after he leaves is probably unrealistic. That being said, however, I don't see there being a huge drop off. My guess will be that the team will move from being elite EVERY year, to being "good" every year, and every now and then rise to being elite, much like most of the "good" teams. So instead of being in the playoffs EVERY year, 3 out of 5 will become the norm, and in those seasons we don't make the playoffs fans will have to "endure" just 8 or 9 wins.
In other words, for fans of the Pats who didn't endure the first 40 years of the team's existence, they will go from being TOTALLY spoiled, to being relatively spoiled.