Lots of companies have been forced to do provide free services to non-customers; consider almost any industry classified as a 'utility'. The question here is 'are message platforms a thing that should be classified and regulated as if it were a utility?'
For me, the answer is clearly no; there are plenty of communication platforms that can be used freely with interoperability (e.g. email). And, as you note, there are lots of interoperable options.
https://w3techs.com/technologies/details/cm-wordpress