You can't. These apps don't federate, and not everyone uses all of them.
As I wrote this comment, I realized I use more apps than I thought.
I use SMS for some folks, Messenger for others, and Slack, Hangouts, and GroupMe for specific groups.
If I'm in a 5+ person group chat, I can't really ask everyone to move to something else. And I want to hear what's going on, so I keep the app around.
These are also all the people who have at least a thousand unread emails in their inboxes. I know one person who has roughly 60k unread emails dating back to 1997 when she created her AOL account. She is not a Luddite and very good at using other types of tech but somehow never mastered email.
There is no doubt, people are going to struggle heavily to catch on to messaging. People seem to just now be getting social media (which is why it is dying) and now they have to adjst to messaging.