Signal's creator runs their own email server, but always points out that 99% of the time, they are communicating with gmail on the other side. This subverts the decentralized nature of email.
XMPP is not a mobile friendly protocol. Even the Matrix.org folks are doing XMPP compatibility, but not implementing XMPP directly. My point is, "don't align with the business of acquiring monetizeable users" is not entirely true, and there are technical reasons (not just business reasons). Everyone with an interest in this problem should be on the same page, and holding overly cynical views about the impediments isn't (IMO) conducive to creating solutions.
We even had a piece of this for a while as Google and I think a couple other messaging providers had XMPP support but it's gradually been killed off.
The "business interests" part of this is that anybody creating a monetizeable user base via a messaging network wouldn't give the opening to others to come in via XMPP. It gives your users a way out and gives your competitors a way in.
No it'd be great but unfortunately the things that make something technically great don't align with the business of acquiring monetizeable users (either directly or indirectly by selling the resulting social graph).
I'm convinced that the only reason we still have email is because it predates walled garden land grabs.