Since Madoka, new mahou shoujo need to be edgy or something. Since SAO, Isekai need to subvert some aspect of it instead of played straight. Now, most of anime watchers don’t like Idol anime. People watch zombieland because it ‘subvert’ Idol genre (or whatever).
If Pretty Rhythm came out today, it might attract unnecessary attention: people who don’t even like idol stuff but having Schadenfreude for having a genre subverted. Basically they like everything else but the idol themselves.

/ai/ are people who like this niche of Idol series with heap of sugar in it. We like it earnestly for what it is.
from the synopsis I guess oshi no ko is psychology anime story, just using idol as possible 'skin'