I really think it's Maxiell's salary that might be holding up Ainge on this one. 5 million isn't a ton of money, but like Davis, Maxiell isn't a starter (or arguably even the first big off the bench) on a championship team. I bet Ainge is thinking that he can get Davis at 2-3 million, get similar production, and save the team some dough (remember, they don't have to pick up Pruitt's option).
And while I'd buy the argument that Maxiell's talents might be a better fit, I'd also question how much it's going to matter if KG, Perk, and Wallace are all playing a minimum of 30 mpg. It's probably not worth the difference in salary for what will transpire only over 5-10 mpg at best.