You are not required to use Steam to buy games on any device (even on the Steam Deck it's trivial to install games from other sources). It has big market power because gamers actually like it and actively choose it over alternatives (often even those that come pre-installed with the platform, the Windows store and the XBox app). It does not prevent devs from selling their games on multiple stores. It's dominant position for sure has market effects, but its a lot harder to argue it has the position for any nefarious or abusive reasons. (Although there are some lawsuits ongoing claiming that they did threaten devs over pricing, if those succeed it might change things)
Oh hey I wasn't aware of that lawsuit and (IMO) that's really good news. My only real concern with steam as a platform was the off-platform pricing clause.
I figured it was likely unenforceable if a studio simply released a "steam edition" or whatever artificial version differentiation, but at the same time they can always refuse to do business with you for any reason (or no reason even). So at that point the clause feels like them making a blatant threat.
People usually mix up things: The clear clause in their ToS is about selling Steam keys. A dev on Steam can generate Steam keys and sell them wherever they want, and Steam does not take a cut on those. For those, there is an explicit clause that says that you need to match the pricing on Steam.
For just selling the game elsewhere, there is no such clause, and there are games that e.g. are cheaper on Epic and explicitly say that's because Epic takes lower fees. But there are also devs claiming that Valve pressured them in private to not do that, and that's what the lawsuit is about.
