Could you provide something that backs up these claims?
I have been using XMPP since 2000/2001. My current address is nine years old (and I control the server). I have a choice of clients on every platform that I use. All my contacts have the same set of choices regarding provider, accounts, and clients. As a bonus, not just my human contacts but also my infrastructure uses it for messaging and monitoring, so I can literally control some of my servers from my XMPP clients.
There must have been at least thirty major IM "solutions" going in and out of fashion during these 16 years. Individually, each might have been somehow "more convenient", but if start counting the cumulative migration effort, I'm not sure the convenience argument holds much water.
¹ I used it in anger once myself, initiated by one of my contacts. It was an interesting experience.
Also, how do you get all your contacts to use XMPP and your server?
It depends on your threat model, like other alternatives.
I have never initiated an OTR session myself, but I have received one from a contact in anger (whistleblowing).
> Also, how do you get all your contacts to use XMPP
In my case, I have been using XMPP for the last sixteen years, so my contacts have "grown organically", to use an en vogue marketing term.
It appears that Mac computers come with an XMPP client preinstalled (and possibly pre-configured?) Linux, the same. Android, there are a number of clients available. Windows, I think there is Gajim, ...
As far as a stereotypical common user is concerned, they are quite happy to install whatever applications their peers are using, which is how they end up with so much cruft on their computers, phones, etc. :-)
> and your server?
Nobody needs to use my server. It is like email, you can use whichever provider you like.
I hope this answers your questions!
Convince them :-) Same as with any other.