This problem is the same. How can Spotify compete with Apple in a completely separate industry like Music when Apple takes a 15-30% cut? How can Netflix compete? Apple is using their position to expand into other areas of business.
This is bad for competition, and what is bad for competition is generally bad for people.
Sure, Apple has a great, safe experience with the store and in-app purchases, but that is no reason to prevent other flows. Why can I not decide for myself whether to use an App’s “alternative” payment system?