To OP's point, you don't have to know the entire 100 hero roster and all of their abilities to have fun, just the general things they can do "Oh I just got stunned by that ability, might want to dodge next time"...
Obviously to be good you need to know them all, but not to get started and have fun at lower levels of play.
Respectfully, OP doesn't know the game. You can indeed have fun playing casually like that, but your win rate will reflect the lack of what you don't know. You can't even see the game until you get some months under your belt, imo. Sound is another underrated aspect: you need to know exactly what spells are going off, how your allies or enemies will position them, based on nothing but hearing it and knowledge of the game.
There's no way I would have learned Dota without a friend to pull me along. It's a big problem with the genre as a whole. LoL is a bit more forgiving and ability spammy, but has all the same issues. And I say this as someone that got near the top of the 2v2 ladder in LoL beta (RIP original twitch malphite combo).
But that's something that MOBAs can do that RTS games can't. Which is another reason why MOBAs ate RTSs lunch. I'm not sure why you're focusing on win rate when there are plenty of people who play MOBAs who are quite frankly garbage at the game(s).
That's a big initial bite to get down.
I played Dota for at least a year until I finally felt like I even understood the baseline. The skill cap on say SC2 is indeed as high as you can take it, but learning the tech tree and counters is comparatively trivial. It's not that Dota is chess or such, there's just a huge volume of material to get through.
But that's also what makes it so rewarding. Dota is the only video game where I feel the same sense of accomplishment when winning as a game of go.
I think the world is wide open to a new awesome RTS game. Just, no one has thought up something good enough.