User blog comment:B214/Light Novel Volume 16 spoilers/@comment-23847729-20140128181036/@comment-1359322-20140129013936

By then it would be kinda meaningless, like i said, Issei would reach a level where he could already be God-Class by the time he graduates high school. By then keeping him as a main character would be kinda stale. Unless if it's like to show events much later after DxD like Issei's family after he marries the girls like how many wives he'll have and how many children he'll have. But even then, that might only be like around the size of 2-3 Short Story combined.