I don't think that is true - success of Skyrim (or other TES series) stand in direct contrast. FPS are indeed non-moddable nowdays but FPS have been done to death and only the franchises (like Modern Warfare, CoD, Crysis) are making enough money.
As per Gibson: "The street find's it own uses for things," and it's that ability of the street to do so that gives long renewing interest into the thing. Either let oneself be reborn again and again, or die of certain irrelevancy.
At some point you have to consider the fact that there really isn't much left to do in FPS. When Quake and HL came we were still in infancy. While CS survived every other mod died a horrible death in neglect. People now know what sucks. And what doesn't has been honed to perfection and there really isn't much left to do anymore.
If you want to see modding don't look at FPS, look at Minecraft.