I mean it's definitely a niche, though we're finally seeing indie Starcraft-descended RTSes trying to expand that (there were many other attempts, but generally in other subgenres): Zerospace, Immortal, Stormgate, Battle Aces.
Unfortunately, most of the other attempts to expand the RTS playerbase were incredibly wrong-headed. Almost all of them were obsessed with simplifying the genre to appeal to more people, but in making the game easier to play they also made it less interesting, which is why it's the big traditional RTSes (SC1, SC2, AoE2, AoE4) that have the biggest playerbase, and therefore the most total casual players.
Unfortunately, most of the other attempts to expand the RTS playerbase were incredibly wrong-headed. Almost all of them were obsessed with simplifying the genre to appeal to more people, but in making the game easier to play they also made it less interesting, which is why it's the big traditional RTSes (SC1, SC2, AoE2, AoE4) that have the biggest playerbase, and therefore the most total casual players.