Mika's "decline" is like the equal and opposite kind of thing that I used to mention RE: Kakko and the PP. I remain convinced to this day that had the Rangers stapled Kakko to PP1 through his rookie season onwards, no matter where he played 5v5, his season would've been evaluated much more like Michkov's currently is than it was. It's just hard to score at 5v5. It takes a lot of problem solving. And for bigger players like Kakko and Mika, that little bit of extra room can go a long way for developing habits (or not developing habits). A younger Kakko needed that space to get touches and work out how to make an impact as a scorer, not to mention, to gain the confidence of his teammates, coaches, and himself as he learned the rest of the NHL game.
Mika... seems like every year he was more and more just killing time 5v5 and had it in his mind that his role was to put up points once the PP started. And more than that, his role on that PP grew overwhelmingly stagnant. Like, nobody moved less in the league on the PP than Mika, except maybe OV. But Carberry even was like, OV, you've gotta move sometimes.
I just think Mika's game got stale. When you get to 28-30-32, that's when guys are less likely to be 'peaking' as far as pushing individual socring records, but they're peaking in a sense of consistency, in maturity, understanding their strengths and weaknesses and how to be mostly good most of the time, as they can be. Some guys lock into that so well they find an even greater efficiency (Kreider comes to mind, Bergeron and Crosby being like the exemplary players of this mold), and other even have new levels to get to that it takes some experimentation and new opportunities to discover (Panarin, I think JT Miller and Blake Wheeler are examples of this as well).
I think Mika's hit that range of his career where he's meant to know what he is and what he's good at, but he kind of got set in a bad mold. He's not a player that can make much of an impact standing still, but that's who he thinks he is. The PP, for whatever reason, just hasn't had it's fastball (he's a part of that no doubt). Just look at his personal heat map on the PP during htat magical breakout, monster season:
Compared to last season:
He's not Ovechkin. That guy can do nothing in a game, and then somehow take one shot and score three goals. Mika needs to be skating. He's not a threat from a standstill AT ALL and he really has never been in his whole career going back to Sweden. I'm still of the opinion there's a seriously good player to be salvaged here but you HAVE to force him out of this rut if your the coaching staff. It's like how habit is just the grooves deepening in the mind, you've got to push him out of his usual spots and put him places where he HAS to behave differently. If you keep him on the PP, you put him in the high slot. At 5v5, I think you move him to the wing for a while. I'd put him on Trochek's RW to make that very clear to him.