It's hard to imagine a devex good enough to compensate for "every so often you need to revisit all the code ever written in this language."

I think this is where concepts from Rust and Go could come in handy. Things like Go’s race condition detection and rust’s compiler validation approaches can be used to statically analyze code. Sure, it’s a meaningful change from how many Python devs approach the language and challenging problem but not insurmountable given the existing work in the field.