Not only are you generally historically wrong, but unless you're on the design team, you're not qualified to make this statement. You don't get to define them.
And to back that statement up, I'll say that MoP is probably the only expansion where most heroics across the board weren't challenging at all if the party was appropriately geared. (and by appropriate, I certainly don't mean raid-outfitted).
BC - needs no explanation
Wrath - most people love pointing out how 'faceroll' heroics were, but that's because they tend to only remember overgearing them. Halls of Stone (Brann) and Halls of Lightning (Loken) produced plenty of wipes. HoR caused a ton of of wipes, and sent folks screeching to the forums demanding a nerf. Oculus needed multiple nerfs and an extra loot bag just to convince people to get through it (and even then, caused plenty of wipes). Trash in ToK caused plenty of wipes.
Cata - again, no explanation needed.
Finally, logic would imply that they actually should be challenging... otherwise normal modes serve no actual purpose.