Why not? Because that is all you really have. Explain how the design and side philosophy have taken a back seat? If anything, the questing and raiding is even more immersive now. You can't regain the fresh, new world feeling anymore that Vanilla or whatever expansion you started playing provided. That's gone. The game has gotten more streamlined, yes, but in the best ways. I feel much more involved in the world with phasing and constant in-game cinematics than I ever did while leveling in BC or Vanilla. And sure, TBC heroics were challenging when you were starting out and maybe even a bit after but that also led to certain classes/specs being useless while others were amazing. You needed 2-3 classes with CC. Mages were god-like and any under-geared group almost required one. Heroics aren't supposed to be challenging. They're supposed to be a step up from leveling and normal dungeons, providing enough loot so you can get into raiding.
Name one boss in BC or Vanilla that comes close to comparing with Wrath/Cata/MoP hard mode bosses. Name just one. I'll be right here, waiting.
And don't you dare bullshit me with some "Activision conspiracy" pseudo-counter crap.