Lolno, it's the other way around, Mythic have been created BECAUSE people rush.
People ALWAYS rushed in dungeon, even back in vanilla (Startholm 45 anyone?), that's what happen when the content is too easy, and you have to do it again and again and again.
Why wast time and going slow when you crush the thing and have done it dozens of time already?
If only blizzard had focused on the "hard" side of dungeon, by making every pack a challenge that ask you to use CC (hey, remember thos?) and careful pull instead of the pack'n'aoe we got now... but sadly that's not the case.
So yeah, I'll blame blizzard on that one, they streamlined the dungeon to make them rushable, so OF COURSE people are rushing them.