User blog comment:Jopjopjop/Volume 18 Spoilers/@comment-74.110.108.150-20140910120029/@comment-24098614-20140911191827

I don't think that Isse's perversion will become less or go away that is like saying that Azazel in the series is not perverted and not desire Harems think of Issei's perversion as a chronic illness that doesn't go away it either gets worse or slightly better, and if you remove the perversion away from his character the sales of Dxd would drop dramatically because then you can call him the Oppai Dragon anymore, and it is his perversion that made him strong.

The reason for his Sacred Gear to evolve is related to his perversion because the original Juggernaut Drive he used took his lifeforce away but his desires and dreams of lust made it evolve, and his chant changed to match this evolution so you might say his perversion is what makes him strong taking this away would make Issei a pathetic and weak character in the story.