Let's not pretend like Trouba is some huge upgrade. He's already made quite a few mistakes, he's frequently behind the play, and his mobility is not good, all of these are bound to get even more exaggarated as he gets older. On the other hand he does bring some physicality and muscles in the crease. Interestingly enough, "everyones" new favorite descriptor of hockey games, Stat cards, had him, from memory, mostly in the negative in the games he played ao far.
As Verbeek said in the intermission of last game, trading Fowler will give younger guys opportunity to grow, meaning that, among other things, it will force the coach to play younger guys more.