Here's the thing. There are other places. Epic, Amazon Gaming, Origin/Battlenet/Ubi, itch, Microsoft store, gog...
Most suck at discoverability or they don't have the variety of Steam. Some are shitty by design (Origin, Ubi, Battlenet) - intended to only get you to play their games. Others like itch aren't built for scaling out to deliver thousands of big games.
This isn't a thing like Apple's walled garden, I feel like this is Steam out performing the competition.
