While there's more choice in today's market, SF is generally far less "niche" than it once was, isn't it? We've seen lengthy and successful overlapping runs from B5, Farscape, Stargate (which racked up seventeen consecutive seasons in one form or another), BSG, a hugely successful revival of Doctor Who that has almost reached the point of eclipsing the old franchise... counting shows like Fringe and The X-Files, I'd say SF has demonstrated a pretty healthy public support base.
It's not that that market isn't there for Trek to take a piece of. The Trek spinoffs ultimately faltered because Trek became niche: they were creatively hobbled by a succession of fatally "safe" decisions and other shows came along and outpaced them, not because SF was too "niche."
The available audience for live eyes watching has drastically changed from when B5 and Farscape were on. People DVR stuff now, stream it on line, wait for it to hit Netflix, download it, watch it on Demand. Many, many people now watch stuff on their own time, rather than when it's actually airing on the channel. Networks like CBS are struggling to get over 10 million viewers now, when B5 and Farscape were on, CBS top ratings earners were getting twice that. NuBSG really wasn't successful. It was critically acclaimed, and SyFy saw it as prestigious, so they let it run longer than they should have, but, it was cancelled with less than 1million viewers an episode.
Yes, some SciFi/Fantasy can get big ratings these days, but, it's typically down to Earth, no major Space opera plots or lots of aliens. It's mostly character dramas, with a little of SciFi trappings that can survive today.
Doctor Who, yes, a very fun Series that actually is
doing really well for BBCA, but, I don't think it does well enough to survive on CBS (And British shows have much smaller budgets, because they don't pay the talent as much as US shows do, and they don't work 16 hour days as a regular situation)