[bluefirebrand]

Steam's customers aren't really the end users, though

Steam's customers are game publishers. Steam provides a service to connect publishers to their audience. Their business model is not "takes money from customers in exchange for goods". It is "takes a cut of each sale that a publisher makes on their platform"

Given that there is no real friction for end users to install Epic Launcher or GoG launcher, is Steam really a monopoly to their customers, the publishers?

If Steam tries to muscle a publisher, they can refuse to publish on Steam and still have options. When popular games aren't on Steam, it does seem like people have absolutely no problem installing another launcher/storefront to access it

Look at the massive success of Fortnite, which is only on the Epic Launcher