RTS did go mainstream and it indeed turned into games very different from old school SC et al.
Plants vs zombies and LoL are the descendants of the genre and are or at least were, HUGE. Tower defense and moba are the two evolutionary paths that RTS took.
Tower defense is super mainstream, but moba, while huge isn’t really mainstream in my opinion. But one things for sure, they don’t have much in common with SC except the lineage.