Thing is, the darkness was always there. When pitching the anime to friends, I often said that it takes place in a peaceful slice of life city that's an oasis from Madoka-level dark outside of it.
I mean there's already the cute mascots looking for little girls to recruit as mahou shoujo. And as much as we take turns hating on Suika she's the norm rather than the exception, systemically. Magical girls gain power by killing demons and I suspect that's what their familiars often want them to be doing. In essence Momo and Mikan are doing a pacifist Undertale run.
I vaguely recall Momo saying it used to be a lot worse and many modern magical girls don't harm peaceful demons. But that background radiation of a very scary setting has been there from the start.
Incidentally, I think this is one reason Machikado Mazoku is such a brilliant story -- there is a lesson here that no matter how scary the world we live in may be, you can create a safe space to enjoy life from. No reason to wait to live your very own slice of life anime story. But such spaces require dedication and protection and the cracks in the shield can form at any time.