Where we disagree is that I think Wagner can still improve / elevate his game and it appears you don't.
I agree he could get a little better by staying another season. My point is that wouldn’t help his draft status. That’s not how the NBA draft works. Staying in school doesn’t help much unless you make massive improvements. Wagner already has the offensive skills polished and there is nothing he can do to make massive gains on defense. Does he need some more muscle? Sure. But NBA teams know he can gain that more easily in the NBA than he can in college.
As a Michigan fan I selfishly wish he would have stayed for another year. Logically it was not in his best interest. There was literally no way he could ever be a lottery pick. He'd have to have upped his scoring to about 20+ PPG, upped his rebounding to about 12+ per game, and all of a sudden become a shotblocker. That sort of stuff wasn't going to happen. From sophomore to junior he upped his scoring and rebounding a little. His pace of improvement wasn't going to accelerate over another year.
And then there's the risk. Somebody like Miles Bridges probably cost himself something like over $100M longterm (when you combine the missed income at age 19 + later free agency + compound interest over time on the percentage he'd likely save) by coming back to school for another year.