Yeah, beyond the "do you really fire your successful coach just to shake things up" question, there is another thing.... most of the best work a coach does is in building consistency and execution through practice. The Celtics basically can't practice, and have by far the most days missed for Health and Safety of anyone in the league.
Sure some of this has been lack of motivation by guys like Tatum/Brown/Smart, but also a lot of it is... young guys and now new guys not sure where to be, guys coming off injury or illness which likely contributes to the lack of energy, and basically being shorthanded.
If this continues well into next year.... then sure, that's when you might decide, you need to fire your good coach because it's easier than trading away your under-performing players.
Also... I'm reticent to dump a coach at the first sign of laziness from players, because it creates an environment where players can slack off. If Brad got fired tomorrow, I have no reason to think the players would suddenly start making the right switches, or pressing harder. Listen, if LeBron wants a new coach, you get him a new coach, he's the best player in the history of the league, he's achieved at a really high level basically every year of his career. If you let young players who haven't done anything run your coach out the first time they don't feel like playing hard, you end up with a toxic lockerroom and chronic underperformers. There are plenty of examples of it.
I think you just write this year off as a unique experience. It's so far from a normal year that you can't read anything into it.