Yeah. RCS requires a data connection -- wifi, mobile data, whatever. Specifically, an internet connection. It is, basically, an internet service.

I can't speak to a lot of different implementations, but Google's "Messenger" app falls back to SMS transparently. Since RCS is a data service, it has the ability to see if the recipient can receive RCS messages and act appropriately.

It's also worth noting that data coverage in the modernized world is pretty wide-spread. There aren't really any more analog cell services, and even the 3G networks are being shut down in favor or 4G / LTE / VoLTE and 5G networks, which are data connections.