If Ainge wants a "star", he has to realize it's a seller's market--there aren't many stars. That means he has to make offers teams with stars will seriously consider. With his stable of middling talent supported by good coaching, I doubt he can attract a star without giving up what other teams covet the most--his high draft picks.
Take a look at Brown at #3, and ask yourself if you'd rather have him than a Westbrook or a Griffin or a Cousins. Sure Brown may turn out to be pretty good, but he's going to have to improve his shooting to be an elite player. And even then, there's no guarantee.
I'd rather have the established star. The known quantity. The draft is a crapshoot--actually a double crapshoot. First the lottery, then the pick. A 19yo raw kid, usually, these days.
A bit like prospects in baseball. Keep 'em or trade 'em for established stars? You win some, you lose some. With an established player, you win.