It definitely would have been nice if they made Kel'thuzad a near-Kael/Vashj difficulty fight - it would have been justified considering the loot he drops.
It would have made killing him all the more satisfying rather than the inevitable mow-down it is atm.  And it would only have prevented guilds from killing one boss, rather than a cockblock like Vael or Kael/Vashj (pre-attunement change), stopping people from seeing later raid content.