Namely - if a user buys Apple products because they want the security/curation/whatever that comes with Apple's walled garden, that advantage is broken by forcing Apple to have other app stores on their platform because now that user cannot get all the content through their trusted source as they could before.
The choice of app store isn't ever in the consumer's hands, its in the corporations hands (Apple or otherwise).
If I can think of that in 30 seconds, then so can Apple. I'll let you draw your own conclusion.
They could trivially do this because as it is, they require that all apps are signed. Irrespective of the distribution method. To sign an app, currently, you need a contract with Apple.
They just need a one line clause stating that you can distribute on other platforms if the app also appears in their store.
This doesn't compute. Does your home cooking tastes worse because restaurants exist?
On one hand you have some happy users and a happy Apple, on the other hand a flurry of penalized users with nowhere to go (we should coin a phrase, like "the year of --linux-- alternative stores on smartphones") and angry service providers.
Up until now US regulators sided with Apple to protect its money, I don't expect EU regulators to be as kind.