Your eleven-nines figure measures object durability, not account survival. It says a stored object won't corrupt, not that you can reach it. A billing dispute, a locked account, a compromised credential, or a provider shutting down takes your original and your backup together, because both hang on the same identity. That's correlated failure, and no amount of prudent layering inside one account fixes it.
And notice where your own argument lands. You say keep local copies and use two providers. That is my position restated. If relying on cloud alone were safe, that advice would be unnecessary. So we agree on the fix, which concedes the danger.