Acquiring James Harden would gut the team, leaving Harden, Tatum, Kemba, and table scraps. The Rockets would want Brown, Smart, and most of our promising young players, plus take most of the next 5 years draft picks.
Were such a trade to happen, we'd be no better off. Or rather, we'd be worse off, because we wouldn't win a title, we'd have no assets for the future apart from Tatum, and Harden would likely bolt after 2 years (or more likely 3 years, because his lard ass isn't going to get another huge contract if he doesn't stop eating, so he may very well opt in on the $47million+ last year of his deal).
If a superstar possibility comes open, there are some that Ainge certainly should consider swapping such a haul for. This ain't it. No one should invest what it is going to take to acquire him vs. what you're getting in return.