You're right in theory, but
possibly not in practice.
Consider two vendors: A and G.
A provides a full walled garden.
G provides a walled garden, but lets you side-load whatever you like, you can ignore the apps in the garden.
Everyone making apps for A puts them in the walled garden, so if you own an A, you buy an app from the garden.
For G, vendors decide for themselves whether to use the walled garden. Many choose to sell direct and avoid both the markup and the hassle of getting their apps approved.
They also get to make even more money by embedding surveillance capitalism into their apps if they sell direct. Or include completely unmoderated and unregulated adtech.
As a user, isn't G better than A, since I have the choice?
It is better than A, provided that every app in A's walled garden is also in G's walled garden. However, if in practice the apps I want to use are in A's walled garden, but sold direct on G to avoid the 30% hit, then in practice, as a user, I am better off with A if what I want are apps from the walled garden.
Of course, I can always do my research to figure out whether a side-loaded app on G is safe to use. But if what I wanted was to buy apps without having to think about them, then I can be better off with A in practice even though in theory, G provides everything A provides, and more.
Now, is there really a problem getting all the apps I use on A from G's walled garden? I don't personally know, since I don't want to go to the trouble of sorting out what is available in G's store versus what I have to sideload. So there's plenty of room to argue that in practice, G is superior to A.
But I do not think it is necessarily superior.