They could definitely just release an app for Android instead of opening the protocol, but as an Android user I'd reject it for the same reason I reject my Apple friends suggesting we all use WhatsApp or Signal: I don't want different conversations living in different chat apps for no reason. That to me is the bad old days of Facebook Messenger+Twitter DMs+SMS where I had to remember which platform each of my contacts prefers to use and then deal with missing features and an inconsistent experience all the time.

As much as I think Beeper's work on iMessage is important, apps like that do not and have never solved this problem. Because then you have different contact identifiers to contend with, the inability to make groups amongst those users, differing features, and the list goes on.

If you look closely at what I'm saying here, it's easy to compare it to what iMessage users say about why Android users create problems for them, and that's true. That's why messaging interoperability is important.