My grandmother lives 700 km from my home, I can't be there every time she changes her phone or breaks something.
I can instruct a non-technical relative to install Signal. Installing Conversations and configuring the right account on the right server is another story.
Maybe https://quicksy.im/ would work in that case. Easy to install like Signal, but it uses your phone number as a routable XMPP address. Unsophisticated users get easy onboarding and nobody else gets locked into some walled garden.
It's encrypted by default. I agree that having no option to send unencrypted messages would increase security somewhat, but the unsophisticated user can always be tricked into sending an unencrypted message. They will just use a different app if something doesn't work. Not really fixable on the application level.
I can instruct a non-technical relative to install Signal. Installing Conversations and configuring the right account on the right server is another story.