I would disagree with everything in the bold. Personally, having watched him and been a fan since he committed to BC, if Hanifin only ever becomes as good as Slavin is now...I'll consider him a bust. It's well below his potential.
I also doubt that Slavin has as much upside over where he is now as people think he does. Go back and listen to what his HS and College coaches say. It's always some variation on "I'm not surprised with his success...he's a smart, mature, poised kid and this is the kind of solid, responsible game he's been playing for years." Give him the eyeball test...he looks like a 4-5 year veteran; he makes mistakes but not as many as the other kids. Slavin plays like a veteran 2/3D because he entered the league thinking the game like an old pro and with above-average talent...but I don't know that there's a lot above that to build into his game.
Slavin's talent is good, but not elite...Hanifin's talent is elite. In the end, I expect Hanifin to end up in the discussion for top-15 defensemen in the league. Nobody sane has that kind of expectation for Jaccob Slavin.
I'd say "Slavin's more than I ever hoped for from Fleury." Slavin makes Fleury possibly expendable...not Hanifin. If I had to make the choice between Hanifin and Slavin, I move Slavin 12 times out of 10.