User blog comment:B214/Volume 23 discussion blog/@comment-172.56.17.58-20170417051436/@comment-10330822-20170419032049

You can be mixed, the house just won't be able to be apart of the 72 pillars anymore, that is what requires you to be pureblood, but you can still inherit a house. Either way, its irrelevant since Rias passed the title to Millicas, who is pureblood.

That is probably what is the most impressive about Zeoticus and Venelana supporting Rias's relationship with Issei anyway, because they'd lose their status if Rias had mixed children with Issei, even if the devil is reincarnated. It shows they support Rias's happiness more than their titles.

That might also be why Rias gave over the title, to make sure the Gremory didn't lose their title of Duke. Now, Rias can live happily with Issei, and the Gremory can still retain their title with Millicas as the heir. And, as B214 mentioned, he is the deserving heir, even if that's not how it played out.

Although, like I said, there's a high possiblity that in the future this restriction to purebloods are lifted, and mixed children can inherit without consquence. What's more, Issei may be in an even higher position that the Gremory are. In short, Rias could have ended marrying up, not marrying down.

This is why I feel there is still conflict to be had with the pureblood-centric devils and pro-reincarnated devils. Whoever has the most say can completely redefine the structure of the 72 pillars, and decide what is appropriate for an heir. Who knows, they may start encouraging bio-diversity like the Asari from Mass Effect. After all, if EX showed us anything, its that Issei's children are incredibly strong, in part, because of their mixed natures.