Given the low likelihood of this happening (for Google, Fastmail, Outlook, etc.) it's really not worth thinking that much about this. My approach is to have a custom domain and forwarding to another provider.
Gmail -> Outlook forwarding
iCloud Photos -> OneDrive
You seem to be only thinking about Gmail. What about the other Google services that would be lost? Anyone who makes their living making YouTube videos will be devastated by the loss of their YouTube account. It can also be a huge problem for those who log into other websites using their Google account.
Gmail -> Outlook forwarding iCloud Photos -> OneDrive
etc