I think my thing with Dubas, and really in general, is where does the responsibility and accountability lie.
It is easy for people to say the fire the GM or fire the coach or make these moves. Sometimes it makes sense, and others it doesn't. I think eventually there gets to the point where Dubas does his job extremely well and still gets ousted just to make a change, but I don't think that happens after 3 years. Not unless he starts being an active problem by making terrible moves, etc.
When I am evaluating Dubas, I look at where the team should be based on what he built. Ultimately, he can't guarantee success. No GM can. He can only give them the best possible chance to win and I don't think there is any doubt in most people's minds that the Leafs should have been able to make it out of the 1st round in at least the past 2, if not the past 3, years we've made the playoffs and likely make a fairly deep run. Maybe it is only a hard fought second round exit or CF exit, and not a Stanley Cup, but we've seen worse teams make it farther than we have.
So I look at Dubas and see that most things have gone fairly well for him. If there was a weakness (top 4 defense, getting some better depth, etc.), he was fairly quick and effective in addressing it. He's taken some risks that have not always panned out (i.e. Sparks), and some more recent moves (i.e. Foligno and signing Thornton) were a little bit of a head scratcher for me, but I can point out any top GM in the league and say the exact same thing. And his replacement will almost certainly not be any different in that regard.
My main concern is that you can do a lot worse than Dubas and likely not significantly better, and even if you do better, there is still no more of a guarantee that the Leafs make it out of the 1st round... And meanwhile we see Dubas go to another team and win a Cup or make the CF two years in a row and hear people are saying "that is what would have happened to us if we kept Dubas".
Eventually, the guys on the ice need to perform and we've seen some guys really step up (Matthews, Nylander, etc.) and others who really have not (Marner, our goalies on occasion, a bunch of guys that Dubas quickly dumped when it clearly was not going to work out for them, etc.)