It feels like the only way to stop this happening reliably, is to get as many people as possible off Google and Microsoft.

I'd argue that self-hosting is the first step. The second step is helping everyone you know get their own domain and host their email for them. If they don't trust you to not read their email on your server, point them to somebody else who offers mail hosting.

The more people we can convince the internet is something anyone can help build outside of the silos, the more likely it is to survive.