You obviously haven't worked on a good codebase. I've worked on both so I know the difference.

Same feature on a good codebase vs bad codebase is like 1k lines of code vs 20k lines of code and is worse in the latter case. Not an exaggeration.

Clean code is real, it's just not what's been peddled to us.

If your code is full of bugs and you ask Claude or Codex to build more features, it will take 10x to 20x the effort because of workarounds and hacks which keep compounding.

If your code has very few to no bugs and no hacks/workarounds, it will also implement hacks/workarounds and no bugs. The difference is pretty insane actually.

You can try it yourself; create a really low quality codebase, full of hacks and try to build features with AI based on its recommedations.