@technomancy I run my own email server. It's nowhere NEAR as complicated as getting fully-interoperable ActivityPub.
Having said that, Google and Facebook killed XMPP with this exact "create product, federate with open servers, accumulate users, defederate, tell users to switch to corporate servers if they want to talk to contacts, pivot to closed app to pull the ladder up" with Google Talk and Facebook Chat.
I suspect that Outlook, sadly, is the levy that protected against this with email.