Koz so we are losing IT (who played incredible last year), Bradley, Crowder and KO and Amir Johnson. 5 of our top 8 rotation players from last season and 3 starters. We replace them with Kyrie for 60 games, Marcus Morris for 49 games and a bunch of rookies and you expect 53 wins still? That doesn't make any sense.
Amir Johnson, really?

We replaced Thomas, Bradley, Crowder, and Olynyk with Irving, Morris, Baynes, and (once Hayward got injured) increased role for Brown/Tatum. Crowder/Olynyk and Morris/Baynes are largely a wash. Irving is an upgrade over IT, and Brown/Tatum over AB is up for a debate. In my mind, however much we lose by having to play the relatively unproven Brown over AB, it's compensated by the fact that having Irving is just plain better than having to tailor the entire offensive and defensive scheme of a team around Isaiah Thomas.
So yeah, while matching last year's win total wasn't a given, expecting 44 wins was pure insanity. Even if Tatum was struggling, this team should have been able to start Morris at SF and win close to 50 games easily.
Any team that turns over all but 4 of their players is subject to exceptional variability. If you thought anything would come "easily" this year then it demonstrates a shocking lack of common sense.
You also missed the important part of his post. Kyrie for 60 games. Morris for 49. Smart for 54. Every single player but Rozier, Tatum and maybe Horford has missed more than 10 games. A bench almost entirely comprised of rookies. When you take all of that into account this team could have easily sunk into chaos. We've had rookies play 26% of the minutes this year. That's an astounding number for a 50 win team and that's due to Brad.

TP Granath,
Usually, when a team turns over +70% (13/17 new players) players it leads to better chances of winning a lottery, than improving on an already high 53 win bar.
I can't remember any similar example in 72 seasons long NBA history to what Stevens (Celtics) did this year.
Correct me if I am wrong.
Cleveland currently has 5 players that were on the Finals team last year: James, Love, Thompson, Korver, and Smith, and turned over nearly half the roster a month ago. OKC only returned Westbrook, Adams, Roberson (out), Grant, Abrines, and Collison (never plays). Similarly, Minnesota also only returned 7 players: KAT, Wiggins, Dieng, Muhammad, Jones, Bjelica, and Aldrich, a couple of which are end of the bench guys that barely play.
Now granted that isn't quite the same as 4, but lots of teams, even very good ones, have a lot of turnover.
Cavs had Lebron last year. Their best player. They have him again this year. They had Love last year. Arguably their 2nd or probably their 3rd best player. They have him back again this year. Plus turnover.
OKC has Westbrook last year. Their best player. They have him again this year. Plus turnover.
MIN had Towns last year. Their best player. They have him again this year. Plus turnober
Celtics replaced their best player with a totally different guy who is now their best player. Plus a ton of other turnover.
What the Celtics have done is much more remarkable. Especially when you factor in all this injury stuff too (fully aware of Love's woes).
Sure, but there are 3 other examples of playoff teams this year which lost more than half their roster from last year.
The 92/93 Bulls won the title, the next season they were a 55 win team with just Pippen, Grant, Armstrong, Cartwright, Paxson, and Williams back on the team. They obviously not only lost the best player in the game, they didn't even remotely replace him and yet were still a 55 win team.
Or how about the 96/97 Lakers. They were a 56 win team that season (53 wins the prior year). They returned only Jones, Van Exel, Ceballos, Campbell, and Blount. Ceballos played in 8 games before he was traded that season and Blount played in just 3 of their 9 playoff games (so was obviously not a main player). Thus, Jones, Van Exel, and Campbell were the only 3 rotation players that went from 95/96 to 96/97.
Plenty of very good teams have had a lot of turnover from one season to the next. Boston has a touch more turnover than most of those type teams, but not so much more that it is outside the realm of possibility. That was the point I was making.