Steam

If Steam suddenly introduced a policy that prohibited devs from selling on other platforms alongside Steam, most devs would choose Steam because they would make way more money on Steam than elsewhere.

The power to do that is monopoly power, regardless of whether Steam is abusing that power currently. I think that their behavior on the whole is pretty good, but that doesn't make them not a monopoly.

You can sell your game for different prices on different platforms, you just can't sell steam keys that way. If you purchase a game on Itch and it gives you a steam key, that's still a steam purchase and is subject to this restriction. If you purchase a game on Itch and it hands you an installer then you can buy that game at whatever price they want to sell it at.

Developers aren’t forced to exclusively ship on Steam or not at all.

That’s just not true in practical terms. If you want your game to be discovered and you don’t have a massive advertising budget, it’s not a serious option to try to forego selling on Steam while staying in business as a game developer. That’s like saying Amazon isn’t an ecommerce monopoly because you’re not “forced” to sell there, even though that would mean bankruptcy and irrelevance for most sellers.