You do realize the MHL has only been around since late season 2008/2009 after a very long while being inactive as a league, right?
Oh, and there have been a lot more than 1 player in that span that spent most of their draft year (undrafted or not) playing mostly MHL and still went on to become good NHLers.
Kucherov, Panarin (played 38 games in the MHL vs. only 20 KHL in '09, undrafted), Buchnevich, Chinakhov, Fedotov, and more besides.
Alexander Nikishin spent all of his draft year in the MHL, went undrafted untill 2020, and is considered now to be one of the best defensive prospects not in the NHL. He's gonna be a pretty clear top-4 defenseman if he ever comes over.
So yeah, the whole "MHL is a weak league" argument to attack Demidov's case as a prospect really needs to be put to rest.
Not saying that Demidov's 100% safe, or that he's for sure gonna be a star, but the potential is there in his case.