@eternaltyro @signalapp @hetzner Last time I looked, #XMPP #MUC had rather evolved group ownership and sort-of moderation features. I'm not sure about the evolution of multi device suppport together with #OMEMO. I think that one was a bit lacking last time. With #conversations_im there is a really polished UX available that likely rivals what #Threema and #Signal offer.
I haven't tracked the state of the XMPP desktop clients, but I guess your mom isn't using any of Signal desktop client and rather just her phone. In general, I've seen all sorts of people get used to all sorts of technical shenanigans, especially if supported by the friendly #familyadmin, if a need is perceived.
But sure, as you say, XMPP ain't Whatsapp (anymore).
Basic point: No real need to sell the alternatives short in times when the primary services aren't available.