That would be true only if a company recreating the same product and improving on it would actually have a chance to compete, which isn't the case. People refuse to have multiple launchers out of principle, having all your games in one place makes it so you don't want to split your library so you keep buying from the same store (sometimes people even brag about paying more just to have a game on Steam instead of an alternative).

The only way to break that monopoly would be for a competitor to come and offer to recognize purchases made on Steam for games they offer on their own platform and to start with, at the minimum, everything that Steam offers and I'm sure I don't have to tell you that's completely unrealistic.