It really doesn't matter who "won" the trade. Success is measured individually and in comparison to expectations.
The real question is "Are we good enough to win a championship?".
SPOILER: no
If you measure the success of trades based on whether it makes you a true contender, you're going to be disappointed 99% of the time. We were never one move away from being as good as Golden State, which is the same situation almost all the teams in the league find themselves right now. Golden State has raised the bar so high that teams with three stars aren't even competitive with them. That's not a typical situation when you look at history and what it usually takes talent-wise to have a shot at a title.
Ainge basically kicked the can down the road slightly while still making the team better for this season so they will still be competitive in case something unexpected happens (a star becomes available, injuries to other teams, etc.).
Oh, excuse me, but if you finished last season with the best record in the east, trade away your all-star PG, hold the #1 pick in a supposedly loaded draft, and invest another pick (potential value: sky high, estimated value: sky high, real value: TBD), then yes, I expect you to be a contender.
Losing to the Cavs in the ECF again, or getting swept by whoever comes out of the west is 100%, definitely, unequivocally not a success, it's a failure of the highest magnitude.
Fingers crossed, eh?