We can argue the return that Vlade got but I think a trade had to happen. Fox was indirectly part of that deal while Buddy, JJ and Giles were direct results. Depending on how Boogie returns from his achilles tear, history may not view this as a lopsided trade. It may even be the case that the Kings end up the winners in the long run.
I suppose the question is whether trades are a zero-sum game. I think the naive answer is "yes" but it may well be possible for trades to NOT be a zero-sum game - perhaps both teams can win, perhaps both teams can lose.
I think it's abundantly clear that New Orleans lost the trade. They gave up Buddy, a first round draft pick, and a second round draft pick to get 65 games of Cousins, and he never appeared in the playoffs for them, and then they lost him for absolutely nothing.
As you pointed out, it's not really clear whether we won the trade or not. The alternative to trading him would have been signing him to a super-max. Then, does he still tear his achilles? If he does tear his achilles and doesn't ever come back to the player he was, then we clearly win the trade. But even if he doesn't tear it, or f he does but he comes back strong, is he enough to be a better asset than Buddy+JJ+Giles+Mason and whatever downgrade from Fox+Bagley we would have had given our expected record with Cousins around? I have no idea. But given our continual inability to win with Cousins, I won't be surprised if the current group, once ripened, will lead us to a better record than Cousins ever did. I think our fanbase generally went into the trade hoping we didn't lose the trade hard, but at this point it looks like winning the trade is possible.