Scenario 1: video calls
I live abroad. The family, including a 80+ grandmother, has Androids, iPhones, computers. If I were to run something, maintenance would be limited to a few times per year, and I couldn't be there when they get a new device. Besides, remote support on android is virtually impossible.
I'm a veteran sysadmin and I have an XMPP ecosystem working with video calls & all, but that is not common. It needs a secured TURN/STUN server beyond a well configured XMPP one, DNS service entries, etc. For someone less experienced I can clearly see that being a nightmare.
Scenario 2: mailing lists
I'd absolutely run to communicate with the group of people I run classes for on mailing lists, except mailing lists are essentially dead since DKIM/DMARC. DKIMv2 might address this, but if you ask people, especially young ones, they'll look at you as if you're from Mars.
Summary
I'd absolutely love to have a non-profit, cross-platform video call app that comes pre-installed everywhere, just as the phone (aka voice) "app" is there, but there's zero incentive from the current ecosystem for such thing. I'm aware of carrier-level video calls, but I seriously doubt it's used a lot.