The code gets bug fixes occasionally because things change (scaling past previous limits, changes in the environment, etc) and of course new features are requir

The code gets bug fixes occasionally because things change (scaling past previous limits, changes in the environment, etc) and of course new features are required once in a while. So the debt usually can't be swept under any carpets. All code rots. If you are lucky to never need to fix it or change it, that's great, but that is (in my experience) extremely rare over time spans measured in decades. Sometimes a code base runs for decades not because it was particularly robust or well-written, but because it is too painful to replace it due to its complexity or the long ago loss of the knowledge needed to work on it.