That’s not how it works. Other apps (ironically including Google’s RCS implementation) use the Signal Protocol. Simply using it doesn’t magically make your app interoperable with every other app that uses it. And Apple would be the last company to go out of their way to make it work.
Nobody here is against open standards or FOSS apps. I am actually lucky/privileged enough to be able to write open source code for a living.
You seem to not understand the reality of the situation and that use case other than yours exist.