User blog comment:B214/Volume 23 discussion blog/@comment-31764731-20170420000500/@comment-1359322-20170421145220

I think it's safe to say they won't run out of Pure-blood Devils even in the far future. There are also Pure-blood Devils who are Low-class and Middle-class.

And this is what Lord Phenex said:

"“You don’t have to say any more, Lord Gremory. It was a good engagement for pure Devils, but it looks like we both had too much greed. Both of us already have pure Devil grandchildren. Maybe it’s the greed as Devils that made us crave for it. Or is it because we saw hell in the last war.”"

If they are really so close to extinction, they would be completely displeased with that situation. They even said they were greedy. This probably means that the Devils are actually nowhere close to extinction but due to


 * 1) the war taking way many of their kin
 * 2) the low birth rate of the Devils
 * 3) they're still in a state of conflict with the Angels and Fallen Angels.

The older Devils became worried and indirectly force the mentality that the children must try to give birth to more Pure-blood "High-class" Devils.

This is base on what Rias said "Big Devils with peerage who commanded about 20 or 30 troops lost most of their underlings because of the war." A troop in Japan definition is 4500-5000 in a group. So it would be about 90,000 to 150,000.

72 Pillars, and lets say 30 Extra Demon. Total 102 clans. The range would be around 9.2 million to 15 million. Then according to Rias and some others, they lost a majority of their numbers and can't form new armies anymore. So lets say 80-90% of the Devils died in the war.

9.2 million's 10% would still give them about 920,000 of Pure-blooded Devils remaining. Then there's the Devil civil war that took place with the Old Maou faction. Lets say it's split 50-50 after that the Devils would still have around 460,000+ of Pure Devils remaining.

Of course it's still a massive drop, it's millions to hundred thousands after all but definitely not enough for the Pure-blood Devils to run out.

But what can run out is Pure-blood "High-class" Devil. There are also Pure-blood Low and Middle-class Devils in fact, those were the Devils Zekram calls "commoners" and do not consider them actual Devils because they're not High-class Devils.

"“I would want all of you who will become the centre of the young generation. The real Devils refer to kins of High-class Devils that has been foretold since ancient times. Others besides that are “servants” and not real Devils. The “commoners” and “reincarnated ones”. Being evil or not will change by the perspectives of humans and other forces. I don’t see the need to become evil. To keep the society of nobles continuing for eternity is what a “Devil” should do.”"

So if they abandon the ideology of High-class with High-class only and give proper chance to those Low-class and Middle-class Pure Devils, they won't be so desperate for more Pure-blood Devils. Unfortunately, this issue doesn't seem like it's going to be solve soon.