The facts are that this team has underachieved in the playoffs the last 2 seasons. I believe that in both cases, BOS lost to good teams, but in both cases, BOS was in position to win, but didn't. So understandably, out come the call to get rid of Mazzulla, Brown, and even Brad Stevens. I get the frustration, but rash fan reactions generally don't align with smart team management.
The Coach: Title winning coach, perennially in the top tier in COTY voting. Overachieved on a historic level in the regular season. Fell short in the playoffs two seasons in a row. Truth 1 - the rest of the NBA world seems to view Mazzulla much higher than the local fans do. There was a lot of "Mazzulla will never win a title" before he won a title, but coaches have a shelf life, Mazzulla's may be sorter than the norm. There is also the argument that his coaching style relative to 3PA is good for the regular season and maybe not so much for the playoffs (except for the season they went in and a title was won). There could be some truth to this point, I am not sure.
The Roster: The roster had plenty of weaknesses that somehow were not exposed in the regular season, but sure were in the playoffs. I am not in the "Tatum and Brown can't play together" club, they won a title. I am also not opposed to rational, well thought out plans to rebuild this roster, that include everything. I would trade Brown for Giannis, for example. Unless the team goes nuclear and trades Brown and Tatum and White, the basic structure is going to remain the same. Two super-max deals and then not enough money to do much else. You can't even afford to pay guys like Kornet ($11M for a bench big). Only teams with stars on rookie deals can do that.
The Executive: Some feel that executive of the year Brad Stevens should also be on the hot seat. Hmmm. I think if Stevens were to become available, there would be 29 NBA teams interested and probably the top 50 NCAA programs. It is not going to be easy to change this roster unless something big is done like trading Brown. If BOS does pretty much anything other than signing min contracts, they end up hard capped at the 1st Apron. You can resign your own players to larger deals, but how does that change the roster?