AB or Crowder for a serviceable (and cheaper) rotation big elegantly solves all the roster problems. FWIW, I'd also prefer to deal AB. This mgmt team has shown they're not too locked in to "positions", i.e., they would have no problem with 3 of GH, JB, Crowder and Taytum sharing the floor for significant stretches.
The problem is that to save enough money in this, you need to trade AB for a rotation big making less than 6.85m, and that list is pretty miserable. Guys like Spencer Hawes, Ed Davis, Brandan Wright, Jason Smith. There's plenty of young guys in that price range still on their rookie deals, but it's hard to find anyone that you'd want to give minutes to that would be available for AB. So it seems like if you start from "AB for rotation big", you either (1) get a really crappy big, (2) add Crowder/Smart to expand the range of salaries you can be looking at, or (3) start including draft pick assets (including yabusele) to get a younger player.
FWIW, I'm moving more and more to the move AB camp. I think Ainge/Stevens really want to look at roster construction as PG / 2-4 mix and match / C (there are some nice quotes out there to this end after they drafted tatum), and AB just seems like the worst fit for that in terms of positional flexibility. Smart's size, ability to post up, and defensive flexibility actually make him a good, if non-prototypical, fit for that. If you move AB, you can still run out:
PG: IT/Rozier
2-4: Hayward/Crowder/Smart/Brown/Tatum
C: Horford/Zizic
Even here there's some flexibility, since Horford can play a stretch 4 role at times, and you can quibble on where Rozier should be, but I think this group maximizes lineup flexibility compared to keeping AB. But again, I don't see a great deal out there for just AB that complements this lineup while also shaving the ~1.9m they need from the cap.