the short answer is that Matt Duchene was traded and everything involving the Avalanche has been magically perfect since then
the long answer is that MacKinnon in his first years was plenty fast and had great hands but wasn't as strong or aggressive. He took a lot of shots off the rush as soon as he crossed the blueline which were not that effective. By the time he took off he was probably 20ish pounds heavier and began using his size much more. Additionally, getting Rantanen as a running mate and having Landeskog as the defensive presence (not infrequently taking center duties) helped a ton because they are also elite and their styles align so well -- they are a significant upgrade from playing on a line with guys like Mikhail Grigorenko and the corpse of Jarome Iginla.
Yeah I agree with pretty much this ^^
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Year 1 he was in a pretty sheltered role with ROR and Duchene on the team and Stastny centering his line (he played mostly as a winger). He still had that crazy speed and especially first step/acceleration that jumps out at you when you watch. He did seem almost like a one-trick back then, he was always sort of "hovering" in the neutral zone waiting for a well-led pass for a breakaway. It was very reminiscent of Pavel Bure on the Florida Panthers TBH (except more in the neutral zone instead of near his own blue line, but Bure played in the time of the trap and 2-line pass so hard to compare). MacKinnon in year 1 basically was a huge breakaway threat, great on the shootout (has since sort of come down to earth), decent dangles, plus player on the PP... otherwise basically a 3rd line sheltered winger though.
Years 2-4 the team around him got worse (lost ROR, Stastny, Duchene declined for whatever reason) and he was asked to do a lot more. Basically he was asked to play like a top-6 forward/top-2 center. It always looked like he was trying to recapture the magic of season 1. Always loitering in the neutral zone for a loose puck to turn into a breakaway. He often looked like he was skating too fast for the situation, e.g. skating full speed in the neutral zone and fumbles the puck looking for a breakaway, skating full speed into the offensive zone only to realize that he didn't have a great angle to the net and his teammates hadn't caught up yet (turns into a low % shot or awkward hook back waiting for teammates).. This was the kind of stuff that worked in a sheltered role but not when teams started keying in on him. I don't actually believe he got any worse during this time (he didn't look like he was getting much better though either to be fair).
Years 5+ I think was a combination of trading away or not resigning players that weren't helping, Rantanen emerging as a star winger (no, Rantanen did not make MacKinnon, it's pretty clear who drives the play when you watch the two) and Bednar/Sakic building the lines and team to best take advantage of MacKinnon's talents and playstyle.