I love the idea of a long-term survival mode too. It's a fantastic idea and one that I seriously considered during the design phase.
There's plenty more planned than what you see now though. Beating the first boss will not be the end. It's just the beginning. It simply the first goal for the player and the section of the game that I'm focused on tuning and polishing during this free alpha version.
Many more bosses, secret bosses, more enemies, and dozens more planets with their own challenges are still to come.