Here's my take - if Apple were to allow third-party payment platforms on their hardware, there are a couple concerns.
• First, I don't know if it's possible to separate the NFC hardware from the Secure Element, which NOBODY should have access to, let alone direct competitors who are desperately trying to scrape your device for every little bit of information they can sell.
• Second, if the SE can be separated and other platforms could be permitted, why would the merchants ever agree to let me use Apple Pay? They get nothing, so I suspect all places that now enable it would quickly shut it off and force me to use something else. This part of it stinks, but I think would be inevitable, and is GROSSLY consumer un-friendly.