Ainge isn't stupid. Whatever his intentions are when drafting someone, that doesn't mean he gets locked onto his initial assessment. He has an idea of what Sullinger is worth. It's probably different from what he projected Sullinger would be worth now when he drafted him. He's too good to salary dump for whatever you can get and not good enough to be untouchable.
Amir Johnson was a gamble that he would at least be a quality reserve worth his contract if healthy with a shot at being someone who could be as valuable as Paul Millsap if given the right role, but with more of his value coming on defense rather than offense. If he is not used in a trade, I think there is a good chance he gets signed to three or four year deal when his current contract expires.
I think Ainge keeps Sullinger unless he gets a great offer and won't hesitate to pull the trigger if he thinks he win the proposed trade. If Sullinger remains a Celtic and enters restricted free agency, Ainge probably lowballs him, matches a reasonable offer sheet, and lets him walk while getting nothing return if some team offers him a stupid contract. There is a non-zero risk of the latter happening.