If it's on DS, Grey would have been released one and a half years after the 3DS came out in Japan, and two years after it came out in the west. It would be roughly similar in terms of timeframes to putting Diamond and Pearl on GBA.
As for Ruby and Sapphire... you do realise that they probably won't be released in Japan until the autumn of next year? I find it difficult to conceive of a mainline Pokemon game being released nearly two years after the successor console has come out, but 3 years would be utter stupidity on Game Freak's part.
Like it or like it not, Game Freak's going to have to embrace the future and move on long before that point. I firmly believe that the cycle that we've had for the past two generations is going to be broken this time out, simply due to the console transition.