Fair points, but I think the number Belichick cares about is 5. One more Super Bowl and he sits alone for most by a head coach.
We've been blessed with the amount of wins we see each year, but continuing this run at 10 wins a year for another 9 seasons is no given. It's actually somewhat far fetched to think this will automatically continue once TB12 retires. I don't think we sink to JETS level ineptitude, but TB12 is a huge part of us getting above 10 wins every year. Over 9 years that's a lot of turn over in players and would be very tough to continue this run without the best QB ever under center.
I think it's possible we see Belichick coach another 4 years or so, and then move strictly into the GM position for a few years before he completely retires in his early 70's. Could be a good chance we see Bill and Tom ride out together and go join the circus like Rock and Mick.