Launch a terrific, convenient new email service that doesn't federate and watch how quickly users will choose federation over convenience.

They did, it's called Facebook.

Slack if you don't like that example.

Those are IM services.

That work asynchronously even when I'm not online, that I can check later, that have functionally replaced almost all of the communication that used to happen over email...

It's technically and functionally identical to non-federated email.

Slack doesn't replace email. That's a silly marketing slogan. Imagine for a moment if it really did. What if Slack became so popular that everyone stopped using email? That would be horrifying. You wouldn't be able to communicate with anyone without using Slack because it doesn't federate with any other service. Every website would have to integrate with Slack for user account registrations. Etc. Decentralized federated communications is the backbone of the internet. The fact that IM services have never broadly adopted federation dramatically cripples the potential of the medium with horrible fragmentation and balkanization.

...and yet, here we are, where most people who wish to contact me will send me a message on facebook before they'll send me an email.

It's not fragmentation if there is a single one that the whole planet uses.