I always felt like GT was a niche game, while MK was more of a massive mainstream one. They are probably talking about the latter games (nfs, burnout, franchises that reached the top but eventually fell off)
I don't think you can be a niche franchise when there are multiple games with 10m+ sales, and even the PSP version broke 4m.
I only ever played the first one myself, way back in 98. I prefer arcade racers to the simulation ones. It's a shame NFS, Midnight Club and Burnout all lost their way.