User blog comment:B214/Volume 21 info & spoilers/@comment-83.7.227.245-20160526135656

For all those experts who have been insanely mad after nerfed Diabolos Dragon you should consider this fact. If that power was on primal level then Issei with combined power of Longinus Smasher, Infinty Blaster and "Absolute Flamethrower"(i dont know its real name) will be on another league for most of the gods if you seek evidence look on spoiler he defeated Apophis who was on par with Heavenly Dragons who have been among 10 strongest beings before sealing. So if he could defeated being on that level then it wouldnt be many antagonists( with exeption for technical masters like Cao Cao however he himself stated that even his technique could be overcomed with abnourmus pwer like EJOD of Vali) and the story should have close end. But Ishibumi have stated that he has another power ups for every girl of Issei Harem so we could expect for at least another 6 or 7 volumes. And going on the future i think that end of DxD would be fullfilling destiny by Issei as the possessor of Longinus which is killing some god from top 10 (probably Indra or Shiva)