While it retained the same core feel and mechanics of the 'regular' game, they had a lot of variety with the different commanders (basically subfactions) that could appeal to different players, including ones that were easier/had less base building. I think Starcraft 2 had the right of it with the endless co-op mode introduced in the second expansion, that thing was very popular with more casual players (it was the most popular mode in the game at least for a while). It's almost like mechanical complexity and base building can be good things! Nearly every RTS dev for the last 15 years seems to beat on this drum of, 'We're not gonna be Starcraft! We're gonna be easier for casual players!' and then they fail miserably and you look at the top 3 RTSes: Starcraft 2, Age of Empires 2, Starcraft 1. One of the core problems is probably devs streamlining away complexity, especially mechanical complexity or base building.

There are some quite cool fan games, especially in the Total Annihilation-like space, but of course they don't have as much splash/marketing. It's not just 'oh I'm cool with AoE/Starcraft and won't try anything else'. I'm on the r/realtimestrategy subreddit a lot, and basically every RTS that flops, well, there are very good reasons for that that you'll see people mention.

AoE4 also had plenty of problems (no ranked ladder at launch? it took 6 months), though it being Relic, that's not very surprising. Company of Heroes 3 has already dropped below Company of Heroes 2 playercount despite having come out just a few months ago. The newcomers to the RTS scene are largely badly made.

they've cleared the local space, like some sort of planetoid, and newcomers have little chance.

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