It's so easy to wait until McDavid/Matthews prove they are legit threats to make that claim. For every one of these cases there's 100 more that didn't pan out.
I see your point though, and honestly it depends on the team. Say for example Vancouver, if they had the choice between Dahlin and Karlsson, it would make sense because not even Karlsson at his prime would fix Benning's team.
In most other cases if someone offers you an under 30 best player in the world at that position for a prospect that has never played an NHL game, you take it and run. The Sabres roster is much better than their place in the standings show, adding Karlsson and a goalie this summer and I don't see how they aren't competing with PIT/TBL in the East.
Your premise doesn’t make sense though. Why shouldn’t, under your logic, the oilers have traded the McDavid pick for Crosby? Like Crosby couldn’t have helped that team just as much as mcdavid did his rookie year?
Of someone offered Crosby for mcdavid, which they did, you should have been there to say “there you have it, McDavid is the 2nd most valuable player in the league” or whatever.
Dahlin isn’t the second most valuable player in the league, but for Buffalo, there isn’t really a 1-1 swap that makes sense outside of mcdavid.