Your physical ability to make actions is not the limiting factor in SC1 or any other RTS unless you are literally in the top 0.1% of players. Anyone who pretends it is shows that they have never played an RTS competently.
More like top 30%, maybe? Once you get to the point of knowing all the upkeep chores you should be doing constantly even exist, doing them fast enough to also have time to control an army or two is a fucking nightmare, and any added stress on top of that like moving workers away from harassment just goes way over the line. The average player cannot attack a base while moving workers away from a liberator while getting upgrades, moving workers from main to third, moving a scout, maintaining production without a queue, building more supply and reading enemy upgrades. Pretending that's just easy casual shit and the only difficulty is when someone is doing that while blinking 30 stalkers individually before they get shot is retarded.
If you're sitting around wondering what to do with your mouse idle, EVER, you're in the bottom 30% for sure. If your mouse is never idle, by definition, the limiting factor is your ability to make your thoughts happen- APM. At best you're arguing that that's due to not knowing technique (tricks with hotkeys and settings that make things require less physical clicking or mouse movement) as opposed to actual reflex and coordination, but that's a flimsy argument- the feeling you get while playing, of being frustrated you can't get your army/base to implement the strategy and tactics you thought of, is the same. It's the same reason people bounce off fighting games when they keep losing matches because they know that a dragon punch counters the jump kick (or slightly later, that you can interrupt a pressure string with it) that keeps eating their face, but they keep flubbing the input and eating shit. Losing because the game because your hands couldn't keep up with your brain feels bad because there's nothing to learn from the experience. There's no 'aha, next time I'll know how to beat that!' moment. There's just butthurt. Luckily, probably more than half the playerbase is blissfully unaware of why they even lose matches, blaming game balance, luck, or a bad build order when in reality their buildings had too much idle time letting their opponent effectively have a head start before crushing them with an overwhelmingly superior army.
It's not really about fast thinking though, it's more about smart thinking, strategically and tactically.
These things overlap. Whether you get there by coming up with 3 or 10 ideas you're taking the best of the lot, and both having better ideas on average and having more ideas help in the end. Again, Bullet Chess (Or speed chess in general) is the perfect example of a REAL TIME game about STRATEGY that isn't constrained by physical ability to implement your awareness or ideas. Card games are in the same space; plenty of them end up being won on time once the game state becomes complicated enough that you're struggling to think of the correct response in time, and responding quickly enough to beat the clock means being sloppy enough to lose on the board.
The most popular online RTS games
Are the most popular for reasons that have nothing to do with gameplay. Don't pretend Blizzard's trillion dollar marketing department and their cinematics aren't the thing that set them apart from the crowd. Starcraft isn't a rose you can 'by any other name'. It's a popular brand. People play it because their friends played it, they saw it first on a video or meme, their parents bought it for them, etc. AI Wars 2 and Offworld Trading Company (Excellent game btw, cool to see it mentioned) isn't an obscure indie title because it has bad mechanics. It's obscure because popularity and quality have nothing to do with each other in the space of video games.
The whole premise of the thread is retarded because it's assuming that the insane boom of RTS and Hack and Slash games following the success of Blizzard was the norm, and not some little industry bubble of everyone and their dog trying to chase the current trend. In that sense, the genre died because World of Warcraft came out, and investors started throwing money at MMOs instead, just like RTS and FPS before that and Fortnite, Dark Souls and Breath of the Wild clones since. FPS popped back up for a while at some point, maybe RTS will as well if some kind of blockbuster comes out, but I doubt it'll be soon, because AoE 4 should have been that game and it frankly shat the bed with bad management on launch regarding patching.
1) Casuls don't play online.
This is true (and many that are just play comp stomps with friends) but they learn about the game and become invested enough to hook in friends from the hardcore players that are terminally online. Nobody is going on forums to post in a 500 page thread about how good the cinematic was after mission 4 of the zerg campaign.