If there's a genuinely good product that's popular because it's good. There's no need to step in and give shittier products more share in the market.

The point in breaking up monopolies is to be more fair for consumers. If you want to say they're technically a monopoly because they have a large share of the market then fine. But I don't see that as a bad thing until it starts abusing its power.

yes, it is "is independent of anti-competitive practices", a monopoly is when there is only one company providing a product or service

Um, there is more than one type of anticompetitive practice? Amazon uses predatory pricing to drive companies out of business, Microsoft uses tying to sell Teams, Google uses self-preferencing for their own services in search results, Facebook acquired Instagram rather than compete with them, etc.

None of which are related to Steam nor has Steam done anything resembling any of these examples to my knowledge.

One of Valve’s favorite anticompetitive cudgels is requiring “most favored nation” clauses in their contracts, prohibiting devs from selling for less on other storefronts (which Amazon also has used).

Valve prohibits people from selling steam keys for less on other storefronts which I think is perfectly reasonable. You can list your game on Steam for $20 and distribute it on Itch for $5 or even free and Steam has zero problem with this, so long as you aren't distributing steam keys via that storefront. This is to try and prevent a developer from leveraging Steam for advertisement purposes but making all their actual sales off-platform.