Maybe I'm lost here within so many comments, but Ian did not defend CD before 2. He actually pitched for 1-2-3K-CD during Origins, but got outvoted. He imagined CD as Eggman trying to undo his defeats in the Death Egg Saga using time travel. What I read from this answer is that it's not some official explanation on why Sonic went to Little Planet. He just said the first thing he thought at the moment of the question. In fact, there was no need for Sonic to go to West Side Island just because he had that vision. He's not searching for the Chaos Emeralds - Eggman is. We don't even know if Sonic knew where West Side Island was, before Sonic 2. At some point Little Planet showed up and that must have caught the attention of Sonic and Eggman, and they both went to Never Lake. A giant natural satellite over an island is not something to ignore. Metal Sonic being the most advanced robot is basically the only argument for CD after 3K. And it's not a strong one. You could imagine Metal Sonic being designed to rival Sonic's speed, while Mecha Sonic mk I was designed to kill Sonic in a fight, and Mecha Sonic mk II was designed to harvest Chaos energy. CD before 2 has more arguments, like Tails being absent, and the spindash being weird as if Sonic was still learning how to do it. But these arguments are not strong either. Tails can be absent in any game (even fricking Sonic 4 Episode I), and the spindash is just gameplay mechanic. The best spindash by far is in Sonic Generations, but of course that doesn't mean Classic Sonic comes from the future after perfecting his moves for years. At the end of the day, CD's placement is a matter of preference. And judging for the new canon/Origins, they went with the idea of the main classic games being a great arc. In that sense, CD before 2 is the best decision, because 3K is a much better conclusion to a saga than CD.