XD wasn't needed. Neither was Mysterious Dungeon. So what? They're used as different experiences of catching Pokemon. XD, a GameCube game that involved a new story, featured new catching methods, and wasn't even developed by Game Freak, cannot be directly compared to a hypothetical G/S/C remake.
What's more, there are technical difficulties in doing remakes in any current gen (third or fourth). This is because G/S/C contain items that don't exist in any other games, even in the coding. Specifically: Apricorns, Kurt's Poke Balls, and the Berserk Gene.
Nintendo has also come under fire because of its common remakes and reduxes. I've talked in very large general gaming forums, and the most common complaint I hear by gamers who don't play Pokemon is that "Pokemon's all the same, they just rerelease the same stuff over and over again." Reduxes are bad enough -- Japanese Blue wasn't needed, Yellow was made simply to sate the innumerable Ash groupies. Crystal can be explained away by the fact that they made such a huge leap in technology (actually connecting a Game Boy to a cell phone, allowing online play years before D/P) that the next games, Ruby and Sapphire, were a step backwards.
The fans as a whole don't want a remake of G/S/C. Certainly, there are many individuals who do, but there are also many who don't, and even more who couldn't care less about G/S/C (or worse, have never heard of it).