A post to Blizzard's World of Warcraft Forums by community manager Zarhym explains why they intend for Icecrown Citadel to be the final raid of the current WoW expansion in terms of tiered progression, even though some high level players complain that they completed all the current raids within nanoseconds of their release. Zarhym makes it clear that it is not in the best interest of the game as whole to spend considerable resources on content that will only be experienced by a fraction of the players in their MMORPG (thanks Ant via IncGamers). The full quote follows.

Compared to the number of players who saw or came close to completing the original Naxxramas or Sunwell, that's actually a promising statistic if you're correct. We've said numerous times we didn't like spending so much development time on Naxxramas or Sunwell so a few thousand players could kill the final bosses. If a handful of players who make up the top guilds in the world, or even top on a single server, are occupied in Icecrown Citadel for at least another two months, you can infer that an incredibly larger percentage of players will find challenges here for months to come. :)