Maybe. At what point do they regret this? I would think only if there are no finals appearance(s) or championships before Donovan leaves or gets injured. The Cavs are definitely one of the best teams in the East now and have the potential to go very far.
They will regret it because Don won't move the needle as much as they hope, and his high usage may attenuate the growth of their youth and limit their ability to grow in the future.
Garland took a big step, with similar growth as we saw with Don early on. Would Hayward have slowed that growth? Probably, and we may see that this year with Garland. Garland is a great playmaker and hopefully for them it ends up being a good pairing. I was expecting Mobley to get more touches this season as a key to the offense, and I expect that to be less likely now. Mobley and Allen are both good, but neither are great screeners (Allen is ok, but closer to Whiteside than Gobert). Don benefitted from more screen assists than any player in the league. It will be interesting to see if/how he adjusts and how his efficiency % are.
I predict Agbaji is going to be a perfect 3 and D player, and expect his shot creation/playmaking to improve even if he is an "old rookie". 6'5" with a 6'10" wingspan that plays lockdown D on multiple positions is the type of SG you need in today's NBA. His floor is high, and Utah will be a perfect opportunity for him to develop on the court to maximize his ceiling. Hardy is going to love him.
Cle will miss Agbaji on the perimeter. LeVert looks to fit the mold size-wise, but he is overrated trash and a horrible defender. CLE will now suffer with a tiny backcourt with subpar D, although they have a much better balanced front court on both ends than the Jazz had the last couple of years, and they have the luxury of being in the weaker East. Garland has youth over Conley, but so far has been not quite as good as a defender (he is a much better facilitator though). CLE will also miss Lauri, who was a good complement to allen and Mobley to offer more perimeter scoring. Whether CLE can get Don to make an effort on D may be the key to their success.
CLE gets a "better" scorer than Sexton in the backcourt they need (LeVert is less consistent than Don, and frankly, may not be in a CLE roster after this year). Pre ACL, Sexton was a better defender than Don, and had similar offensive efficiency %s on a MUCH worse team. The injury didn't give him a chance to grow and jell with the team this last year, but he was quite efficient on both ends on cellar teams. If he can recover and continue to grow, the Cavs may have already had the backcourt they needed to take the next step.
The post-ACL ability will be a key driver, but Sexton is young enough that he should be fully recovered.
As the Jazz push for a quick rebuild with a splash in next years draft, Agbaji will be the prime Danny Green type player we will need.
I actually think this trade will end up paying the most dividends for Utah in the future. Some of the late pics may be gold, and I think Don, after being so close to going to NY, will still want the limelight of NY or Miami, and will jet when his contract is up.
I watched a lot of Cavs games last year, I liked what they put together. They were having a lot of fun on the court and playing good team ball. It will be interesting to see how they adjust. I was looking forward to see them with a healthy Sexton this year to see what they could do, but am glad this trade happened, as I predict it will be great for Utah.