I don't think that Steam will expand the audience of the simulation. This game is too serious and not everyone has enough patience to get through the learning curve, for example, it took me two days to learn the RMS grappling technique with Challenger STS-8, now I am getting ready for Hubble launch STS-31 mission with Discovery.
If you have a look at other serious simulations, many of them are distributed also directly from developers. Steam is more for mainstream gamers, for example Train Simulator fans, get into the train, press two buttons and start rolling, then purchase multiple addons. Meantime space flights are the pinnacle of human invented technology.