That you believe it's possible means that you either don't understand how strongly protocols impact UX, or you don't understand XMPP limitations well enough, or you don't understand SimpleX differences well enough.
It is simply impossible to build the product we have, with the qualities we already have, leave alone future qualities, based on XMPP.
SimpleX is build around a very different concept of identity than XMPP has, and this is the core of XMPP that can't change.