No Coach lasts forever, eventually the players tune them out.
Look at the bums rush Bill Fitch got here from many of the players after winning a title. They got swept by the Bucks. Then KC took over and they won two more.
Look at Heinsohn in '77-'78, Cowens walkabout undermined him and he got the axe after two titles.
Doc lost his right hand - Coach T a couple of years ago, KG and PP are declining, RA who might have always been the 3rd/4th wheel is gone. Rondo just does not seem to be a leader. Perkins dealt away and if he had stayed he would have faded away. Too many new guys plugged in at once.
It's not as rotten a mess as it was in the 1978-1979 year (Rowe, Wicks), but it's a lot like the 1990-1991 season. 3 old war horses, 6 pretty good players added in, a nice regular season record, but a 6 game loss to the Pistons in round 2. The players had talent, but didn't often gel as well as they should have, in particular in the playoffs.
I started writing here about blowing it up after they lost the Finals. It would have beeen sad and ugly, and the results would not be sure, but we'd be likely closer to the next title than we are now.