If you go through your contact list right now, how many people are on iOS, and how many of them do you think you could successfully convince to use XMPP as the primary method to reach you?
Oh sure, but it's still a counterexample to your statement. I can convince people to use XMPP, and almost nobody is using Matrix if you don't do the convincing.
I don't know iOS users, except one, who already used XMPP. Most people I talk to on a regular basis already use it. The ones that don't either don't bother with apps at all (my grandparents), or are not close enough / frequent enough contacts to bother with anything beyond SMS.
