> If the market wants this kind of experience, then surely it will continue to exist no?
That is the battle Epic is currently fighting, to remove the ability for anyone on the market to offer this option.
By requiring everyone to allow third-party app stores, you effectively remove the option for a fully curated experience. Because as mentioned, once a few popular apps start demanding usage of the third-party app store, consumers will no longer be able to choose a fully-curated experience without foregoing those services. And that will lead down an inevitable loss of privacy, increased demanding of permissions for marketing, etc - all of which Apple has fairly successfully fought back solely through their centrally-moderated experience. We all lived through the early days of Android before the built-in flashlight when you had flashlight apps demanding full permissions to monitor everything you do.
It is, in a sense, another "paradox of tolerance" - you have to restrict user freedoms in one area (what users are allowed to install) to protect them in another area (predatory developers). Apple's walled garden is the only thing keeping those marketing departments at bay, their restrictions on app-developer (and ultimately end-user) freedom are what protects your privacy. Similar perhaps to how GNU and BSD regard "freedoms" differently - BSD regards developer freedom as paramount, while GNU regards user freedom as paramount, and these are not quite the same things. While BSD protects developer freedom to release proprietary code on top of BSD code, GNU restricts developer freedom and forces that to be released under GNU license as well. And ultimately that translates into a loss of user freedom in some sense - surely users would like the ability to have ZFS distributed alongside the Linux kernel, but GNU restricts that freedom in furtherance of the big-picture.
Ironic as it is, the Apple Store is the GNU of this situation. They are willing to burn some specific cases of application developers who are not willing to accept the licensing terms, in order to protect the big-picture of user freedoms (privacy). Android is the "BSD", where they are more concerned about developer freedom than user freedom. And the loss of specific apps is analogous to the friction between mixing source under copyleft license with other open-source licenses.
If you want third-party app stores, Android exists. This is about Epic fighting to take away the option to have a moderated, quality-controlled experience and eroding the ability of users to maintain their privacy.