This is just three big corporations fighting over their respective slices of the pie, if you think any of this is being said or done for your benefit I’m sure Epic has a plentiful supply of really tasty Koolaid for you. But no pie, sorry.
I never thought about it like this. It could be viewed differently because the Xbox is a game console and the iphone is a general handheld computer, but perhaps it should be illegal to restrict users installing software on your device by any means they choose, though there's no reason for you to support those means.
It's like claiming that nobody can have a monopoly on electric car charging stations because the customer could just buy a gasoline powered car and electric cars don't even have majority market share. It's still a monopoly. It's a monopoly on charging stations, not a monopoly on cars.
Any mechanism for side loading apps would also be an Apple product, designed and written by them. They would be responsible for supporting it, and ensuring it was secure. Maybe they don’t want to do that, so who gets to force them to, and who gets to decide if they complied with that directive? Who gets to specify it and take responsibility if it causes problems and incurs costs on Apple or issues for their customers?
You’re not talking about stopping Apple from doing something, you’re taking about coercing them by legal requirement to do new different things, and you’d better be very specific and careful about what you are forcing them to do.