I think it was Boeing who actually worked this out at some point in the '70s or '80s: when you need to make changes to more than X % of the software, you're bet

I think it was Boeing who actually worked this out at some point in the '70s or '80s: when you need to make changes to more than X % of the software, you're better off rewriting it from scratch. I don't remember what the X was, and I can't find the reference right now, but I want to think it was somewhere in the span of 30–60 %.