Comment by detaro

Nothing "ostensible" about PC being an open platform.

Any PC will run any number of game stores. Steam is large despite not being the one owned by the platform maker and installed by default (Microsoft Store and the XBox app).

Steam does not prevent publishers from selling in other stores too, nor does it enforce pricing outside its store. (E.g. there are games that are cheaper on other stores, citing Steams larger cut as the reason)

It also allows publishers to force users to install the publishers store to play games sold on Steam. (See e.g. Ubisofts launcher, being required to install games and selling you additional subscriptions without a cut to Steam)

To a limited degree Steam even lets publishers use Steam infrastructure for sales outside Steam, for free. (Publishers can sell Steam keys for free on other platforms, but the number is limited and here pricing has to roughly match Steam pricing)

Even on Steam Deck, the only PC hardware where Steam is actually the preinstalled default store, running games from other stores is supported and the main inconvenience to it is that most stores don't have a supported version for it and you need to use third-party workarounds.

Being an open platform doesn't ensure a market doesn't have other weirdness going on (in this case, there being a strong consumer preference towards Steam, certainly in part due to Steams existing large position, but not only), but its a different thing and most of the usual competition law approaches don't apply. Steam is popular because Steam is popular, not because Steam is using its strong position in another business to push its app store business.