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The interoperability thing is a trash idea from the beginning
Proprietary messaging is worse though. Email is interoperable and relevant as ever. Just nobody bothered to seamlessly apply PGP for encryption (probably spy agencies actively worked against that too).
Why?
Imagine secure apps being forced to give some parts of their users data to proprietary apps. Besides its too tricky to implement, like every app would be forced to use the same protocol as the proprietary app (specifically in messaging)