Best answer. If the money is ok, and the environment not too toxic/stressful, you might just see it as a challenge to secretly improve a codebase without anybody noticing, while still delivering what the higher-ups want to see. Or maybe just scratch the first part and try to see how much further you can push that turd with every coding crime imaginable. One-up the juniors in ugly hack Olympics. Ship a feature and put it on your CV before leaving.

Otherwise, walk away immediately.