If I'm "forced" to sing a song with screaming in it [maybe it's a Spotlight song? or it's helping me get a trophy, somehow?], then hopefully it's charted as a talkie and I can sing freeform and with no screaming at all. I'll do it in the style of Richard Cheese and Lounge Against the Machine.
Doesn't even have to be talkies. Even if screaming/growling/etc. vocals are pitched, you don't have to sing it as such (and in most people's case it would be easier not to try to match the singer's vocal style).
The bigger issue is people just not liking that style of music.
I think you see a case where hmx didn't put out much in the way of those metal genres, so the only way for "metalheads" to get their fix for those genres was to go to rbn.
It sort of created a self feeding system and lead to a lot of those songs being higher than you might expect because hmx simply wasn't filling that need.
Meanwhile, unless it was really popular bands/songs, not much non metal floated up because hmx had most of that covered with regular dlc. You didn't need to get your rock/pop/alt fix from rbn.
But I don't expect hmx to suddenly release a bunch of metal because a bunch of metal is high on the list. They know what sells to the masses, and rbn was getting a fraction of the sales regular dlc got. Most of those metal songs aren't going to be top sellers when put out as regular dlc.