Most of you know I buy into the whole "story has been told, leave it be" argument, but I got to thinking recently... would Kawamori have been better off saying they settled somewhere distant in the galaxy, and that is that? By saying they disappeared, he is stirring up some controversy. People will debate it and continue to discuss it because he left them with an ending that is a mystery. Fans want to solve mysteries, its in their nature!
Hell, he probably should have just left Flashback 2012 as the ending, and not said anything else. They flew off to explore the universe, THE END. Not like he can follow up now anyways, with Hikaru's voice actor being dead.