It does have a niche audience in general, yes. However, we have seen evidence through the years that it is capable of breaking out beyond that niche audience in certain cases.
TNG was that, for example. People today forget just how much of a success TNG was. It had massive ratings, rivaling prime time network shows, and had a big impact on pop culture of the time.
Voyage Home was another example. It achieved huge box office numbers (for the time) and had great success outside the normal sci-fi market. To the point where it became a cliche to talk about people who say "I never watch Star Trek, but I've seen the one with the whales."
I'm not saying all Trek can do that, or should. I'm just saying that it is possible to make Trek that appeals to more than its normal niche audience.