If you were going to do it, for a coach you need someone who can guide Tatum and Jaylen to the next level (leaders of a championship team) while at the same time carrying out the GM's plan to round out the squad. If that means they're sticking with Nesmith, PP, Langford they better be able to develop young players too - which would rule out someone like Doc.
For a GM an upgrade on Ainge would be someone who can hit on non-lottery draft picks and find some under-valued vets (like say a Seth Curry) who can come in and strengthen the roster. That's been Ainge's biggest failing. I do wonder at times whether the game has passed Ainge by a little bit. He's only 62 so that shouldn't be the case but maybe he's not as into it as he used to be?
Bottom line it's easier said then done. Were ownership to do it, they'd really have to get it right or this whole thing could easily collapse and morph into a total rebuild.