> To use iMessage, all you need is an Apple ID, which you can create for free. To use RCS, you need to have a phone number, which means that you need to have an account with a carrier. You also need to have a phone to use RCS: you can't use it on a tablet alone without LTE.

I’m under the impression you need Apple hardware to use iMessage, and that even third-party solutions rely upon proxying via real Apple hardware which has to be turned on (I get the impression some basically offer the hardware as a paid service, and others require you to own the hardware?). This tips the balance completely the other direction, as a lot more people have phone numbers than can use iMessage without further expenditure.