Why would I want a man in the middle of my communications?
I've used all-in-one messengers before. They come and go like the wind. You'll be downloading a shiny new all-in-one messenger periodically anyway.
Just trading one kind of inconvenience for another.
And still, periodically changing an all-in-one messenger is still better than keeping and changing half a dozen messengers.
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[0] - Whatever is the most appropriate webdev word of the day.
Once again, the problem is not technical, it's political. Until interoperability is enforced by government, I expect companies to keep building their siloes. Network effect and tragic commons and all that.