Was this trade made for 3-4 years into the future? When you make a deal under the notion that it is to improve your team NOW, and it does exactly that (to a degree even greater than you expected), worrying about what happens 3-4 years into the future is stupid. As long as you scout and draft players well, those minor league pieces are replaceable. 3-4 years from now we may have prospects even better than Barreto, at which point are you going to be crying that he was dealt for a 7+ WAR third baseman?
If you want to argue semantics, theoretically we could actually trade Donaldson now for even more than what we gave up for him. Why? Because he's actually gotten better, and while I'm sure that when we acquired him there still might have been some doubt around the league that he was "playing over his head", his season this year likely cemented the notion the he is in fact a true superstar player on a hilariously team friendly contract. The point? Not only are we "winning" this trade NOW (Donaldson's value greatly outclasses the value produced by what we gave up), we could very realistically move him for assets that are "expected" to outproduce the package that we gave up for him. Under that scenario, we'd be winning in the "future" as well (as long as those said players actually produced at/near expectation). That is even looking past the fact that Donaldson in "3-4 years" could still easily be outproducing what we gave up for him by himself.
Trying to paint this trade in any sort of negative light is flat out stretching to find some sort wart. You can find a "wart" in any good situation if you dig deep enough. You won the lottery? Maybe you get so caught up in your new lifestyle that you die of a cocaine overdose which was fueled by your new-found wealth, I guess winning the lottery wasn't so good after all?