They could open up iMessage, which they originally said they were going to do, then stopped talking about it entirely. Even if they didn’t want to open it up they could make an iMessage Android app, like their Apple Music app.
[1]: https://www.reddit.com/r/apple/comments/1xuzif/what_ever_hap... [2]: https://www.engadget.com/2017/10/16/apple-faces-440-million-...
It tough to blame Apple for not open-sourcing a protocol they are sued about by a patent troll. Until resolved properly this would be a toxic code-base anyway.
Of course they could restart from scratch, but I guess they have no incentive to make efforts about that. What will be meaningful however is be what they'll do after patent exhaustion date of infringed VirnetX "patents".