one theory I am thinking through is that people say AI is good at greenfield and has a harder time editing code in a legacy code base.

So maybe you just treat projects as greenfield instead of editing.

Like if you want to make changes to a page you generate a new version of the page instead of editing the old one in place.

Then you keep the old version as fallback if problems come up with the new version.

When code gets complex its much harder to treat each request as greenfield. You would need to rebuild dozens of features into the new page every time.