"Even if Apple allowed sideloading, I don’t trust Apple to come up with an elegant solution, though. They will put every warning they can to discourage users from sideloading applications. It could make the user experience miserable, worse than it is on macOS. Why? Money is at stake here. A lot of money, actually. Because Apple seems to be run by lawyers and greedy people, we can expect everything."
They could then warn users their battery life would be worse, cos 2 persistent connections, and make it scary. Meanwhile I, as an Android user would happily take up the offer.
I'll tell you why: money.
The hidden challenge is tracking connection tracking timeouts in stateful firewalls and NAT gateways, so you can keep connections alive with the minimun of data exchanged. This is like if ISP A has a clean network, you only need to ping once an hour, but any packet sent will be recieved without delay; but ISP B needs a ping every 60 seconds or further packets will be dropped without notification. This isn't that hard either.
Bandwidth of push messaging is nothing compared to app downloads, and probably more of the effort would be on attracting quality applications and vetting applications and maintaining business relationships.
All that said, Google lets non-play apps use Google push, as long as the phone has play services, not sure why Apple would do it differently.
Cannot have it both ways. If it will be possible, FB will use it, and write a detailed sideload instructions.