I don’t see how that is an argument against ”Yes, Apple can dictate terms in its own store the same way that your local grocer can.”. You don’t have freedom of choice as to who to buy from at your local grocer, either.

I think that, in principle, companies creating a market should be able to decide what gets sold there and demand any cut of sales they want.

That also is what I think laws say in most countries. It’s fairly normal with game consoles, for example, and I expect you can’t sell merchandise from studios competing with Disney (if they still exist) in Disneyland or sell ice cream there without Disney taking a significant cut, or sell Shell products at an Exxon pump.

The big question, though, is whether the market power of a few players in mobile apps is so undesirable that action is necessary, and if so, whether existing laws are sufficient to correct the issue, or whether the rules of the game should change and if so, how.

I think law will be changed, but how, I’m not sure about. Any law saying “this applies only to companies Foo, Bar and Baz” will be shut down in courts, so arguments must be formulated as to what exactly is wrong with the current situation, and decisions will have to be made as to whether those rules also should apply to, for example, game consoles.