Why can't apple publish an iMessage app for linux, windows, and android? Telegram and signal have no trouble maintaining applications for this, and they've got far less money than apple does.
RCS and SMS have been a total mess, yes, but every other chat protocol I've used has been better than iMessage in terms of supporting cross-platform communication. It's only iMessage which fails at this fundamental part of being a communication app, that of being available on multiple platforms.
I know you're going to say "the reason is spam, you need to pay apple $700 to get a device capable of iMessage, and they can ban by device, which deters spam"... which okay, fine, make iMessage be a $15/mo subscription to use on any non-iOS devices, that'd solve the spam problem just fine while still letting android users join back into the family group message chat again.