JR came in and hit homerun after homerun in what I will always say was sheer, absurd, dumb luck. But those first 18 months or so were as good a run as GM has ever had for this team, maybe league-wide. His moves and decisions from the 2nd Cup onward were so absolutely stupid, random and rudderless (his baseline throughout his career) that I'm convinced he just lucked out, simple as that.
I think luck plays a large part in GMing. Outside of objective idiots like JR, Hextall, Chiarelli, and Milbury just doing stupid shit all the time, lots of moves around the league are just "well this guy's a solid player and seems to fit, but it's up to luck whether he does or not". Comes down to unknowable shit like a player's comfort level, how they gel with their linemates (Sid + Hornqvist were made to be linemates but couldn't stand each other, etc), coaching, etc.
If it's Tulsky, I hope he's as good as people think he's gonna be. If it's Dubas, I hope he learned a ton from his time in Toronto. If it's anyone else--bummer, but we'll see.