They are currently a very benevolent monopolist: they don't seriously compete with publishers that use their platform. They only occassionally produce a great Half-Life or DOTA game as a some sort of "halo effect" demo to encourage the industry to move forward (e.g. to show how VR is done right with Half-Life Alyx). Avoiding releasing Half-Life 3 might have been good for their neutral role. They are like a TSMC of game distributors, who explicitly promised not to compete with their customers.

They remain one-stop shop for games. The gamers don't have to weigh down their systems with a bunch of different bloated game stores, each "needing" several GB of memory occupied at all times and adding minutes to machine startup.

Not to mention conflicts between a bunch of different anti-cheat and anti-piracy technologies on the same machine. And stores with bad UX (e.g. that make you suffer additional wait screens or naggings; Steam is as unobstructive as it gets).

Compare that to Netflix: at the begining, it was a "one stop shop" for movies, it had a lot of classic movies and TV for reasonable price. When the concept was proven, the production houses revoked the license for content. Competition came to the sector and each movie production house set up their own subscription based streaming service. You need to keep paying a whole bunch of services if you want to be able to keep up with all popular TV production or watch classic movies.

So, the competition would in theory be great. But even if stores didn't have subscriptions, the gamers don't want to fill their personal data, check privacy settings, configure, troubleshoot, debloat, ... a different app store for each game. When people have a slice of time to play, they don't want to deal with all of these.

I'm sure that when Valve forgets what keeps them successful and start becoming a Netflix, the Steam store will get replaced.