Sometimes, you can improve the software best by patching, sometimes by reefactoring. And eventually, as it gets older and bad traits accumulate, it becomes easier to do The Big Rewrite. That doesn't mean that it is impossible to fix the legacy code, just that at some point you're better off starting from scratch, but with the old one as a pattern to go off of.

Aging and reproduction feels like the same pattern. Sure, you may be attached to the legacy code, but sometimes it makes more sense to do the big rewrite instead. Legacy code performed well for its time, now it is time to retire it.