Not really a sunk cost fallacy. Existing code needs to be maintained. Some of it should be deleted since it costs more to maintain. Some of it shouldn’t be deleted since it might bite you in the behind when you realize that all of that code was correct (although gnarly) and now you’ve introduced regressions. And which code is which? Hard to say.

Sunk cost (fallacy) is about making decisions based on things that you have already lost. But you haven’t lost or expended the code—the code is right there, and it’s hard to know if it’s more of an asset or a burden.