That’s exactly the problem. They only allow this one venue to get apps.
Nobody is arguing it shouldn’t be possible for users to restrict themselves to the walled garden. But for those users who prefer choice and competition over nanny Apple exclusivity, there should be a sideloading option. Just as on the Mac. No Mac user would ever argue the current situation with MAS as well as independent dev distribution is untenable. It’s just fine. And if it works on my Mac, it can work on my iPhone.