Removing Heirlooms will not curb how quickly dungeons are completed. Player knowledge of both dungeons and characters simply leads to things dying faster, its as simple as that.
One suggestion I have heard voiced in this thread is automatic mob scaling that adjusts to player or average group item level. In this sense, whether you are in a party full of heirlooms or greens, the pacing and scale of the dungeon should average out.
Of course, it would be on Blizzard to scale things properly and do away with the ridiculious speed at which dungeons can be completed now a days. I do fully agree though, dungeons are way too much face-roll at this point. I don't want them lasting hours of course, but finishing them off is ~5 mins certianly doesn't feel fun to me either.