It's very attractive to assume most or all of the problems can be blamed on one person.
If that's true, the problems are much simpler to fix.
Unfortunately the Celts problems extend beyond the coach, or specific players, or the GM.
It's a lot of things in confluence. There is no easy fix.
All of that said, I still don't quite get why people think that the coach is the sole person responsible for poor shot selection, poor discipline, poor energy etc. The coach isn't playing a video game. This isn't 2K. The coach doesn't control the players once they're on the floor.
That said there are plenty of valid criticisms of Brad, such as his inconsistent rotations and sticking with veterans like Teague well past their obvious expiration date. I don't think that getting out-coached by Erik Spoelstra is a particularly effective criticism considering Spo is one of the best 2-3 coaches in the league.