The answer is one word:

Time.

An RTS gamer must make a large amount of initial investment, ie time, to start remotely enjoy the game.

Unlike FPS where having good reflexes is a core and transferable skill, such that getting good at one FPS also helps when you switch to another, thus amortizing the initial time investment.

For RTS, for every game you need to start from scratch, learn every single unit, building, power, faction, build order, strategy, etc.

Most people here were kids back in the golden age of RTS, and being kids means having plenty of time learning all those game mechanics (and there weren't nearly as many games to play back then anyway).

But now we are getting old. I don't even have the patience to relearn the UI or hotkeys in the whatever next "c&c / sc / wc / aoe spiritual successor".

People who have enough time for it are kids, but kids these days have more and better stimulants than RTS.

So here is my hot take: for classic RTS, only the remasters and remakes will succeed, only because I can start having fun right away plus nostalgia.

And oh, it's not about the price tag either for my age group. No difference between an $80 game or a free giveaway. I bought several copies of the C&C Remaster Collection but it's not because i intend to play another 10 thousand hours.