Thing is that "the next game is probably not Terraria 2" is a different proposition from "there will never be a Terraria 2". It's perfectly possible the ReLogic team would like to do something different for their next game, and come back to Terraria with a Terraria 2 after that next game, for example. Whatever their next game is, whether it's Terraria 2 or something different, we can expect that they'll keep supporting it with updates for a long while before going on to their third game -- Terraria has been going on for ten years after all.