If the groove and polecat have niche markets what makes you think every mold doesn't? People throw different things and like different molds. Yes, there are your eagles and teebirds that are in almost every bag on the course, but that doesn't make other molds superfluous.
On top of that, if a disc is still being produced, it's because people are buying it. That means it's not superfluous to some set of disc golfers.