I have several friends who used to work at Valve none of them hate the place, they still have friends there, etc. But they tell similar stories as to why things that normal companies do successfully are impossible at Valve. Perhaps it’s best summed up by something one friend said about her year and a half at Valve: “I first learned who my boss was on the day she fired me.”
Google tried this, notoriously dense grating and then firing basically all the managers at an all-hands. That didn’t work out well at all... And now they have over-steered in the opposite direction!
The platform it runs on (Windows) is open, unlike the App Store. Competitors exist on said platform, including a store & game pass run by the platform owner.
The fact that Steam still runs the show is a testament to their ability to just do things better than anyone else. Sure, there is a sort of network effect at play, but there is no other “moat” here - let alone a monopolistic one.
That said, the PC gaming landscape has completely warped around Steam. Epic had to offer huge incentives to get EGS exclusivity deals. Smaller games struggle without a Steam release, and even big companies with their own storefronts have decided the sales boost from Steam overrides the 30% cut Valve takes. And Steam is so entrenched at this point that it's difficult to see how a competitor could make a meaningful dent in Steam's market share.
Despite this, if we're going to have one dominant PC gaming storefront, Steam is probably the best we could hope for. Despite my many misgivings with Valve and Steam, it's difficult to imaging the situation improving if the dominant platform was run by a company like Microsoft or Epic. And it's fair to say that PC gaming wouldn't be nearly as big as it is today without the success of Steam.