The ideal outcome of Hayes would fit really nicely next to Brown and Tatum. He has high level court vision that would stir the offense and create a lot of open looks. He's big and strong, which fits well in CBS switchy defensive system. He can create his own shot one-on-one either with step-back threes or getting into the mid-range which gives another option at the end of the shot clock. Tatum would be the first option still, but you need a couple guys who can get their own shot.
He fits the timeline well and could be an alternative route financially, as his contract would be better than Hayward's or Walker's for the next four years at least (I'm not eager to get rid of either. I like them both and want them to retire as Celtics, but I'm just pointing out the alternative route).
It all depends on how you perceive his ideal outcome. If you think it's something like Luke Kennard, it's a bad trade. If you think it's something like D'Angelo Russell, it's still a bad trade (although some might disagree with me). If you think it's something more like Luka/Harden-lite, then it gets more interesting.
On pure value, a first team all-defensive player is probably worth about that value to a contender, but few contenders ever have that high of a pick. On subjective value, I really think Smart's role on defense could be mostly covered by CBS' scheme, Brown, and Langford. I know he's dynamic defensively, but I really think the team defense would not see that much of a drop-off over a full season. Although the stats sometimes disagree with me, I think his role on offense could pretty easily be replaced by a rookie like Killian Hayes, or simply by giving more touches to Brown, Tatum, Hayward, and Walker.