>there's no reason to assume the average author would magically create fewer bugs than the original OS library authors initially did
Have you read this old code? It's terrible and written with no care at all to security often in C. AI is much much better at writing code.
Do you have a specific library in mind? I think it would have to be an ancient, unmaintained C library.
But I think most OSS code isn't like this -- even C code born long ago, if it's still in wide use, has been hardened by now. Examples: Linux kernel, GNU userland, PostgreSQL, Python.