Apple should port Messages app to Android and other platforms, this is a great opportunity for them to capitalize on. It is unlikely though, as the strategy is to get people to buy iPhones to get Messages.
As a user I wish that these apps were widely available because I prefer them to alternatives, but as a shareholder I'm not at all upset by this. At least Apple is charging for these privacy-protecting apps, which is the most important detail about WhatsApp: it was already profitable charging users $1/year before acquisition. If Apple ever does release FaceTime and iMessage for Android I hope they charge subscription fees for both.
Most iOS users get annoyed at the android users when they get green bubbles, instead of getting annoyed at Apple.
As long as that continues to be true, I don't see iMessage ever coming out for Android.
...and all that would fit on something like a Raspberry Pi 3B+ nowadays...