What? The collection is primarily based around the anniversary of Halo 2 which, on November 11th of this year, will have come out exactly ten years ago. And much like Combat Evolved Anniversary, will have a fully updated and remastered campaign. 343 went all out on the campaign, hiring Blur to do cinematics, going to Skywalker Sound to do audio, etc.
It's irrelevant what fans will be buying it for. Because the argument would still be valid if there was no Halo CE, Halo 3 or Halo 4 in the title. The whole point of this collection was built around Halo 2 Anniversary and the updated campaign and possibly multi-player.