The Z-Crystal is the only thing that's somewhat comparable, and even then, it's not as big of a deal as a complete new form. The only problem with the Z-Crystal is the fact it can't be transfered to other games.
All those Pokemon you mentioned were somehow avaible by other means(Munchlax is in SM and Beldum is in ORAS as you mentioned, the event giving it early doesn't change that).
Torchic is more comparable, with Mega Blaziken being a new form but here's the thing: Mega Blaziken requires an item that isn't ingame and can ne put on any other Blaziken, meanwhile, Duskroc is an evolution, one that needs no item, an ingame mechanic and it already has two other forms that evolve normally, not only that but you're stuck with thag one specific Rockruff if you want Duskroc. It's contradictory to the other two Lycanrocs and counterintuitive to anyone who wants it.
It'd be fine if it was an item you could get that would then trigger the evolution ala Gligar or Sneasel but nope, it's this weirdly evolocked Ruff that is limited in time and quantity