This argument doesn't make sense. Many developers are de facto forced to distribute their apps on iOS. There are only two mobile platforms globally. Deciding not to support one of them would be economically (and in some cases functionally) unviable for a very large number of apps.
However, the counter argument that opening iOS to other stores and payment methods would not affect users who prefer the App Store is not necessarily true either. If developers can choose not to distribute via Apple's store to avoid restrictions that are unfavourable to their business model then users would no longer be able to buy those apps in the App Store.
This is the dilemma that needs to be solved.
One solution could be to adopt a rule similar to the one for social logins. If an app supports any social logins at all then it must also support Sign in with Apple. Unfortunately, adopting a similar rule for the App Store is a lot more complex.
If an app rejected by Apple is then not allowed to be installed via an alternative app store either, Apple would once again be able to veto apps for whatever reason they want. And if developers were free to set a any price they want for each store, they could effectively make the App Store unviable.
I still feel that there is a set of rules that could make this work. The complexity is unfortunate though.