User blog comment:B214/Light Novel Volume 16 spoilers/@comment-23847729-20140128181036/@comment-1359322-20140128181343

Nah, by the time Issei finishes high school, he would be very powerful already. By then it would be kinda meaningless to push anymore further.

Also DxD isn't ending soon, there's still Volume 18, 19, 20, etc.