Germany and Switzerland have some exotic secure messengers that were quite popular (and may still be) like threema.
There was also some quite suprizing stuff. My inlaws were, until recently, using Viber. They didn’t know why (none of them are computer people). It was wild to me to discover a chat app I never heard off in a circle of people completely outside of tech.
Anyway, the only longterm solution is an open standard. Propably XMPP. Or email after all (it works, look at delta chat).