At the moment, correct me if I'm wrong, as it stands, we don't know for sure if Crono and Marle ARE dead, just that they disappear. Anything could have happened at that point and I think even the Japanese are leaving it open at that point so they can have a storyline thats easier to continue on. If they DID kill of Crono and Marle, there would be a lot of pissed off fans that might not play any new Chrono game that comes out, and they're trying to make sales, not lose them.