That is also my experience. Doesn't even have to be a 10 year old codebase. Even a 1 year old codebase. Any one that is a serious product that is deployed in production with customers who rely on it.
Not to say that there's no value in AI written code in these codebases, because there is plenty. But this whole thing where 6 agents run overnight and "tada" in the morning with production ready code is...not real.