I agree Starsector as it is now is not ready for Steam. It does not take long to reach endgame, and there is no win (or lose) condition. Also, people will need to install 64-bit Java to play more than a few mods. If Starsector needs mods to be good for more than a few games, it will not last long.
The masses will be attracted by a shiny new game for a month or two, then move on to more fresh meat... unless the game is superb enough to be replayed over and over again. Starsector, good as it is, is not at that level yet.