If Apple were to open iMessage (both to other platforms and the spec/implementation) they could own the messaging market almost over night.
Of course there is no real benefit for them in doing so though. They have no interest in being the dominant messaging platform. So it will almost certainly never happen :(
For now it seems the best option is Signal as it falls back to standard SMS if it cannot send via IP. Of course you lose the advantages of E2EE and all the other lovely features of Signal but is it at least graceful for the user experience. Wire appears to be just as good as Signal with the exception of it not falling back to SMS. If you don't want that though I seems to be down to personal preference.
Of course there is no real benefit for them in doing so though. They have no interest in being the dominant messaging platform. So it will almost certainly never happen :(
For now it seems the best option is Signal as it falls back to standard SMS if it cannot send via IP. Of course you lose the advantages of E2EE and all the other lovely features of Signal but is it at least graceful for the user experience. Wire appears to be just as good as Signal with the exception of it not falling back to SMS. If you don't want that though I seems to be down to personal preference.