I think if someone were to try scifi as a subscription based model it would have to be a Battlestar Galactica series or Stargate or another big franchise with a built-in audience such as the next Battlestar Galactica: Blood & Chrome.
I think you might be the first person to say BSG and SG are big franchises and Star Trek isn't. In fact, if we put you and Petey in a room together you might annihilate each other.
Even Enterprise showed Star Trek is at least as big a franchise/draw as BSG/SG. I would argue it's still larger, but the difference we're talking about is minor. Don't get me wrong, I think it will be incredibly hard to launch a new show online. But with Temis arguing that Star Trek could attract 1m viewers to Showtime and pay $15/month subscription, I would think that Star Trek could draw viewers to an online first run show too.
I'm not saying the finances make sense yet, and I doubt CBS would want to experiment with a marquee franchise, but if a soap can be financially viable online with soap viewers, I would imagine sci fi can't be too far behind.