Non-technical people (friends and family) seem to be fairly comfortable using Conversations on their smartphones, and converse.js seems quite user-friendly in a web browser. But it indeed doesn't always work, and there are people with whom I "only" communicate via phone, email, and in-person.

Wasnt Google Talk XMPP?

It was highly successful and used by ordinary folks all over.

Yes. Google's longest-running messaging service, according to https://killedbygoogle.com/

Also they don't advertise it, but it's actually still running... just no new features, no compatibility with Hangouts/etc. and of course they disabled federation.

That depends on the client; Quicksy (https://quicksy.im/) makes contact discovery pretty easy.