The Middleton->Kuzma trade feels very much like an "end of life" contender trade. The kind of trade a team that once was a contender makes when they are desperately trying to hold on just a little longer, squeeze just a little more out of their run.
Middleton's bets case is clearly better than Kuzma's bets case, but it was becoming obvious they were never gonna get Middleton's case again. Kuzma is at least an inning's eater, somebody who can give you competent play and take some stress off an aging roster. He's also been a fairly decent defensive wing in the past. So its not a terrible swap, its just something that's unlikely to move the needle.
The end result is going to be a rebuild, they are moving closer to that conclusion.