If Ainge brings AD in he's basically swinging for the fences. If ad comes in, we don't win a championship, then ad walks next year pretty much everything he's done since trading kg and pierce is rendered meaningless.
The thing is, if you take Ainge and co. at their word, they are all about winning titles and nothing less.
If the goal is truly to compete for a title at any cost, then it makes sense to go all-out to add an All-NBA 1st team / MVP caliber player. Davis is exactly that.
So if they don't go all-out to trade for Davis and to keep Kyrie, then what was all that Danny has done since 2013 for, exactly? Ostensibly, the plan was never to build a fun young team with a deep and balanced pool of talented players. The plan was to compile assets to take a home run swing when the right guy became available, even if doing so was risky.
If they genuinely want to do anything it takes to compete for a title and nothing less is worth accepting, then if they don't get AD, they're probably looking at a major rebuild anyway. They might as well just plan to go for a full on youth movement. If they're serious about their "title or bust" attitude, a youth movement isn't much better than starting over completely.
Worst case scenario, they're sitting here a year or so from now with a relatively clean cap sheet and all of their own future picks controlled (assuming they don't throw their own picks into the AD deal). That might not seem so much worse than having to lock in to the next 4-5 years with Brown / Tatum / Smart et al.
I'm not saying that above is the best way to look at all this, I'm just saying that if the team truly only cares about winning titles, this all makes sense.