OP did not say that it would affect the quality of the Apple app store. The specific quote was: "Apple has vested interest to keep apps made available to be clean and not going to compromise iPhone".
Allowing third-party app stores effectively means providing a vector to bypass the official app-review process, which is the cornerstone on which the permissioning system rests. If you allow third-party app stores, then any sufficiently-popular app can create their own "app store" and demand that you give it full permissions, which they will turn around and use to scrape data. It will become a race to the bottom, permissioning systems cannot stand if the app-review process does not exist, and the app-review cannot exist if third-party stores exist as a mechanism to let anyone publish anything.
Facebook has already gotten their hands slapped for exploiting sideloading to mine user data, and if they could spread it to the general user-base then they will do so immediately.
So your question about "how can an alternative app store affect the Apple app store" is a red herring, and not at all what OP said - what OP said was "allowing third-party app stores will degrade the experience of iOS users" and that is absolutely true, Facebook has already tried to do this before and this will allow them to take another turn at the ring.
Allowing third-party app stores effectively means providing a vector to bypass the official app-review process, which is the cornerstone on which the permissioning system rests. If you allow third-party app stores, then any sufficiently-popular app can create their own "app store" and demand that you give it full permissions, which they will turn around and use to scrape data. It will become a race to the bottom, permissioning systems cannot stand if the app-review process does not exist, and the app-review cannot exist if third-party stores exist as a mechanism to let anyone publish anything.
Facebook has already gotten their hands slapped for exploiting sideloading to mine user data, and if they could spread it to the general user-base then they will do so immediately.
https://arstechnica.com/gadgets/2019/01/facebook-and-google-...
So your question about "how can an alternative app store affect the Apple app store" is a red herring, and not at all what OP said - what OP said was "allowing third-party app stores will degrade the experience of iOS users" and that is absolutely true, Facebook has already tried to do this before and this will allow them to take another turn at the ring.