The summary suggests that the desperately-needed answer to the issue of people being split between multiple communication platforms... is another platform. I probably don't need to go over why that doesn't make much sense but, to summarise: not everyone would go over to it, and then you've just introduced one more circle to the diagram.
Personally, my approach would be an umbrella app, linked to whatever existing platforms you use but abstracting out the particulars, and configurable based on the user's priorities (security, functionality, speed, cost etc). When you want to talk, you add the people (potentially setting some other parameters as well), and it intelligently decides what the best platform is to send your message/host your discussion. "Oh, you want to have a personal discussion with Roy and July? I'll use WhatsApp, they both use it and respond quick.Oh, you need a confidential business discussion with Paula, Derek and Sam? They all use email, but that's a low security channel: do you want to use it anyway, set up a Slack channel with P & D and invite S to join, or Slack with P & D and send a separate message to S by Yammer?"
While universal IM clients go some way towards this, the next steps are to group contacts across services for individuals, start including none-IM contact methods (email, SMS etc), and to pull the decision of how to contact people from the user to the client. That said, I'm out of date of UIMs, it may be that some already do some/all of this?