There's a lot of merit to what has been said here. Brad is, generally speaking, a strong NBA coach. There's no denying that, his record of getting teams to the playoffs with gritty defences and balanced offences is testament to that very thing.
However, he has been at the helm a very long time now. He is currently 4th all time in games coached for the Celtics, and if he sees through his entire contract extension (through until 2025-26) he'll end up coaching between 900-1000 games total, second all-time for the Celtics. That's a long time to be at the helm in any organisation. I've always thought that when someone is in a coaching position for too long, particularly nowadays, there can be some sort of fatigue with listening to them. Especially if they're not raising championship banners. I know it shouldn't be the be-all end-all measure of success, but I would be shocked if Brad sees out that contract without winning any championships.
I'm willing to give him until the end of 2022-23 before fully diving into the 'Fire Brad' camp that some others are in, but I've certainly been closer over the last 8 months than ever before. I need to see what he does in the playoffs differently to last season and this season. I want to see what he does for our offence now that it's healthy - a team with Tatum, Brown, Kemba, Fournier, Smart and Parker on its roster should be a potent offence. I want to see our defensive intensity lift. I want to see some rotations that make sense!
Yet this season is very hard to pass judgement on. Ainge botched the roster management, there's no two ways about it. He's (in my opinion) offset that a little by adding Fournier and Parker for next to nothing, but even those additions have been disrupted by COVID. Players have played with sub-par effort on numerous occasions, etc. I feel like this season more than any other in NBA history is hard to really judge coaching.
Patience is probably the key to assessing all this. We've gotta see how it all goes in the playoffs and next season. If nothing changes, then the pitchforks will likely come out, but I'm not going to make the mistake of underestimating Brad Stevens or the Celtics