Earlier quoted context omitted.
If that were the true reason that Apple is forcing Appstore, and that's a big if, then Apple could still offer this "amazing service to customers", by contractually forcing companies to always also offer payments via Apple if payments are accepted elsewhere. If I can think of that in 30 seconds, then so can Apple. I'll let you draw your own conclusion.
If the app is distributed via a non-Apple app store, then there is no contract between the app developer and Apple. What mechanism would Apple have to enforce that policy?
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.