I agree with a lot of this post, but disagree with a few points.
The Pats will be contenders next year and actually I think this loss fires them up. As you said, Belichick is at least sticking around as long as Brady is there. I also agree there aren't any other true AFC powerhouses.. yet. Although, if Houston could actually keep their key players healthy throughout the season, they could be very dangerous.
But I am not buying into Brady playing at a top level until he is 45 unless I see it. It's already amazing that he is playing this well at 40, but time catches up very quickly in the NFL. Manning had a 55 TD season and two years later he was clearly done. Brady is going to get hit more as he slows down and those hits are going to add up very fast to an aging body. Personally, I think he has two more seasons in him as a top level QB, which is still incredibly impressive.
As far as replacing Brady and Belichick, good luck. Two of the greatest of all time are not going to be replaced easily at all. Before the Garoppolo trade I thought Belichick would outstay Brady, but now not so much, unless they draft a top level QB soon. If Belichick does stick around after Brady. I think the Pats will be fine. Maybe not quite as dominant, but they won't disappear. If he's gone, expect there to be some dark periods.