I don't think FG popularity is waning so much as it has been in one of the periods of being "cool" and now things are going back to a state of population stability. There has always been a (hard)core group of riders who enjoy the benefits and simplicity of FG, and periodically the general cycling community notices them and sees them as "cool". Then after putting one together and riding it for a while, the "normies" either convert it back to multi-geared or sell it to someone else while proclaiming all of the benefits of riding one (such as "learning to spin circles").

It's a great example of Stephan J. Gould's theory of "periodic equalibrum". The core FG group rides around barely being noticed by other riders, then someone spots the single cog and excitedly discusses it wth other "normies", then there is a rash of enthusiasm in the general population that changes the scope of the FG population for a period, then the "normies" seek out the next "cool" thing and the core FG population stabilizes again. This is how younger riders get involved, to replace the older riders that die off, all the while maintaining the size of the core FG population.

The same thing happen with unicycles......