User blog comment:B214/Volume 23 discussion blog/@comment-185.112.105.215-20170419115644/@comment-1359322-20170421065805

I don't have much time so i'll cut it short.

"“The newly produced Devils—. The ones like your servants, the reincarnated Devils, are expanding in terms of strength but that would make us, the High-class Devils with old history background, lose our place. There are old nobles who gets close to the reincarnated Devils because they are very powerful. Well, that might be alright. The newly produced Devils are also important for our future. But we can’t allow the pure-blood Devils to go extinct right? You and I were chosen in order to prevent the pure-bloods from going extinct. My house is safe because I have my older brothers. But there are only two siblings in your house. And your brother is someone who left the house. Then there would only be you, Rias, who can inherit the House of Gremory. If you don’t take a husband, the House of Gremory will go extinct in your generation. Are you trying to crush the household that has been continuing for a very long time? Because of the past war, there is not even half the number of Devils who were referred as “72 pillars”. This marriage has the future of the Devils on the line.”"

I think Ishibumi retcon this part by introducing Millicas since he realises if there's no Pure-blood in the Gremory they'll be considered "extinct". Anyway the main point is the bold part. Ise isn't the only one who got close to nobility others did too. But the ones that did get close are either


 * 1) Concubine
 * 2) Didn't marry the main family but a close relative to the noble.

I can't imagine those nobles being willing like the Gremory to let a Reincarnated Devil lead the clan after all.

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Also i explained the entire situation already didn't i.

http://highschooldxd.wikia.com/wiki/User_blog:B214/Volume_23_discussion_blog?page=2#comm-85436

So it doesn't matter whether or not Ex is a Pure-blood or a Half-blood. Millicas is ultimately the one to inherit and Millicas' children will be the one to succeed him.