>they want to eventually disable or turn off matrix.org.
What happens to everyone on matrix.org?
Disabling registrations of new accounts, not deleting old user accounts.
However something that they are working on (which is a fairly complicated project) is making accounts migrateable between homeservers. Then, users would be able to seamlessly migrate their accounts off Matrix.org.
Also, Matrix is definitely about more than just not-XML - the entire protocol is set up as eventually consistent sync of rooms between servers, which they said would have made a mutant XMPP if they had tried to shoehorn it in