They have a monopoly but that’s because they’re just the best service. It’s not just that they were the first, they’re just consistently the best. Everybody who has spun up another ancillary launcher and DRM service has always made an inferior product to Valve’s.
Epic games bribes you with free games, launched without a functioning cart, and hoovers your data.
EA has gone through Origin and the EA app, both of which are awful; Origin being the butt of jokes for years and the EA app being an unstable piece of garbage that logs you out every day with “a particularly annoying bug”.
Ubisoft. Self explanatory.
I could keep going on, but Valve earned their position in the market. Could they reduce the cut and still exist with a good profit? Absolutely, but that’s the only thing I’d really want them to change - treat the devs a bit more fairly.