Maybe we don't need it, but there are new Pokemon fans every day. We've played the game, and now it's their turn. Remember, its been almost ten years since those games were originally released. These 6,7,8,9,10 year olds may have never gotten to play those games.
But the bottom line is these series makes money. Its about the money. If they can update a game and put a full price tag on it. They will.