As far as how good the Celtics are next season, I want to be optimistic but 50 wins is pretty high. 50 wins is good for the 4th or 5th seed, which I'm not sure is going to happen because I'm pretty certain the east is going to be better this coming season. The east didn't have Paul George for the whole year and Chris Bosh for about half of it. They're going to be back this coming season and should pick up where they left off.
Cleveland, Miami, Chicago, and Milwaukee I am 100% sure will be better than the Celtics. That hurts the 50 games case. Atlanta and Washington lost valuable pieces but they will still be fighting as much as they can. They won't be as good as they were last year but they'll still be up there. Those 6 teams, I believe, should be better than the Celtics.
We also have to factor the west, who currently have 6 teams (Spurs, Warriors, Clippers, Thunder, Rockets, Grizzlies) that have rosters constructed to win a championship, and they are going to treat every game like its game 7 in the NBA finals because they can't afford to lose homecourt advantage in the west.
I hope the Celtics play well this year, but 50 wins I think is too high of an expectation for them, and, not to sound like a Debbie Downer, but it is possible the Celtics might not make the playoffs this year. I'm not counting on it but it could happen.