User blog comment:B214/Volume 23 discussion blog/@comment-31764731-20170420000500/@comment-23853100-20170421173851

Did you not read what B214 just said? He just pointed out a major flaw in your theory. Your theory only applies to the Pure-blood Devils that are of nobility status. There are Low-class Pure-blood Devils too. The maids and butlers of the Gremory could be Pure Devils too just not of nobility status.

Now the issue with the Underworld isn't running out of Pure-blood Devils but majority of the nobles discriminate against the "commoner" Devils, in short Pure-blood Devils without nobility status and low demonic powers.

If the High-class Devils are willing to accept change and actually stop discriminating against the Lower-classes, then the Pure-blood Devils would continue to exist in fact, it wouldn't even face any issue of running out. Commoners > nobles in numbers.