You started talking about the problems with LD, implying the trade matters in that context, as in Sergachev would help with LD issues. Well he wouldn't because he is bad right now. Also leaving out that we also had bigtime offensive problems last season, and in that context picking up a skilled 50-60 point young forward helped filled a need. No he's no perfect but that does contribute to scoring depth which was severely lacking but now is pretty good at 5 on 5 (PP another story...)
And yes I like to evaluate a trade like this in the present and immediate future, because trying to project out potential and upside 5 years is just silly. When comparing two young first round picks, progress is from prospect/fringe NHLer to established impact player is very important and Drouin is miles ahead, he has accomplished this while Sergachev has not, that matters, a lot.