User blog comment:B214/Volume 22 spoilers/@comment-73.46.6.122-20160819203110/@comment-9595671-20160820010855

Well, I can say Issei and Rias won't take up the positions even if they could. Why that's the case? Because... the Four Great Satan page states they can't have harems, the job means sacrificing his dream which as EX shows he achieved.

Sona and Seekvaira as the one commenter said likely lack the strength. Plus I'm pretty sure Seekvaira doesn't want any time taken away from her mecha fascination.

Sairaorg on the other hand could, but due to what Zekram said in Volume 18, I doubt he will. Due to people like his father in the Bael household, he likely knows can't let his clan have more authority than the Satans if things want to change for the better.