User blog comment:B214/Season 3 confirmed!!!!!!/@comment-25072311-20140628063828/@comment-16281343-20140628092503

The sucking in Qwaser is essential to the story, the same as pressing nipples in DxD.

So, that cannot be replaced even though that was what Ishibumi originally wanted to do. The story has progressed so much that replacing pressing nipples to sucking would be change future scenes in the story.