Niche is unrelated to the length of time a series lasted. Generally it simply means that something is understood to be a subset of a larger market. Something can appeal to a niche audience for a long period of time, and people can make careers at it as well. For example, Model Railroader is a niche product but it's been around since 1934.
Niche also doesn't mean that a work of art is flawed. A TV series made for a niche audience can still be awesome, as fellow fans of Community and Firefly can attest.