To add to this, even if it were really fully open, like, say, Lemmy is, because it requires servers there’s the issue of being allowed on someone else’s server and whether servers are modified, and whether server owners want to interoperate and so on.
In some ways the RCS debacle has been similar to the Fediverse debacle about federating with Threads, or with undesirable servers. Even if the protocols are open there can still be bad actors.