How can Capela be a bad fit for this team?
There is no team right now. We have 2-4 guys you can say with any confidence will be around in October.
What if they are planning on signing, drafting, or trading for a playmaking PG? If you had a real lead guard on this team, along with Smart and Hayward, that's several good passers who could create easy looks for a guy like Capela, while Tatum and Brown or Brown replacement X score from the wing.
We know nothing right now. Danny is in chip acquisition mode, so he's probably not worried much about fit.
Aside from your first question, everything you say is correct. There is no team and we have no idea what Ainge has planned.
That said, Capela isn't a great fit for this team and many others because he is more of a traditional big than the Celtics have utilized since Stevens arrived. He is, at best, a passable defender at the perimeter and does nothing to stretch the floor. His inability to shoot FTs also limits his value as teams will hack him hard when he does get the ball down low.
Lead guards and good passers are great but teams will not double, trap or blitz any of the guys you've mentioned to the same degree that Clint Capela has seen over the past few seasons in Houston. Ainge may have some magic yet to do but finding floor spacers to surround Capela with isn't going to be easy without cap room and/or trading away one of the core guys who are left. As such, absent the Celtics changing their style of play to be post-centric, Capela's best attributes will be wasted on this team.
More to the point, the guy is under contract through the '22-23 season and while its not egregious, its a lot to tie up in a player who has limited application in a lot of current NBA team's systems. In other words, if the C's get him, he likely won't be an easy to move "asset" on the follow.
I don't get the fascination with the guy overall - he is great in the right set-up but he really doesn't move the needle for most teams and is relatively expensive.