You seem to forget a couple things:
Teams were doing everything they could to hack and slash and obstruct MacKinnon and his linemates, because you can still do that in the playoffs for some ridiculous reason. Also, when the centers behind you on the depth chart are Tyson Jost, J.T. Compher, and Pierre-Edouard Bellemare, you can focus your efforts on just the one guy and not suffer much in the way of consequences. That, and the fact that we know MacKinnon was not 100% healthy played a big part in his inability to score consistently in the postseason. Does it entirely excuse him? No, of course not, but until the Avs figure out how to put some better talent behind him on the depth chart, that's going to be the number one reason why they keep bowing out in the 2nd round.
And chill out on the "playoff performer" thing. It's as much a product of dumb luck as it is effort and talent. Saad most certainly earned a lot of those goals but he was also shooting at a ridiculously high rate, like 24% or something. I'm sure you recall the goal where he actually whiffed on the shot attempt and it ended up fooling Fleury? Yeah, Saad deserves props but if you're actually trying to put him on a level above Nathan MacKinnon, you're out to lunch.
Saad is a secondary scoring threat. If he's the most talented player on his line, chances are that line is going to do very little. He also isn't the hardest worker. I didn't say he was Pavel Brendl, but you can look it up yourself--he's not the most driven either. That's one of many reasons why, despite his phenomenal talent and vision, that he's not a legit star in his own right. The Blue Jackets traded for him expecting him to be a core talent, a few years later they traded him back to Chicago, and word was it wasn't an amicable parting. He can say whatever the f*** he wants, doesn't mean he knows the winning ingredients to a championship team more than anyone else.
I wish the Avs had been able to retain him because he's a really good player (and yes, I have no doubt he would've signed had they offered a satisfactory contract), but I can kinda see why Joe wasn't thrilled about paying him long-term.