So, Apple wants to control what people can run on their devices and Facebook wants to track their users. Both are right about the other on these topics.

As soon as Apple allows / is forced to allow other app stores, if ever, Facebook might tell their users "for the full experience, install Facebook from this alternative app store that allows tracking". It seems to me there's no good ending for this story.

The choice for me is obvious and simple. Avoid both of these companies.

Every corporation that gets too much unchecked power will eventually abuse it. Why should Apple get a free pass?