I don't know if road cycling is waning in popularity. Mountain biking or bikes that can go "off-road" are certainly growing popularity, (and price.) So there is likely some folks moving from the road to the trails. Are they being replaced? The cost of cycling, in general, continues to go up way faster than inflation. Hate him or love him Lance's success in the TdF was a major influence in the US of the increased popularity of road cycling. Now that he is fading further into history and with taint to him does that too have an impact?
How impactful is the location where people cycle? I live in Southern California near the ocean and the Santa Monica Mountains. So all year round riding weather, a huge variety of choices of terrain with and without cars, (or few.) Take that and compare it to areas where comfortable weather is only half of a year and choices of riding conditions are very limited probably makes a relevant geo difference in regards to the OP question.
Sorry to be redundant but the cost of road cycling is getting insane from the bike and equipment to maintenance and riding apparel or equipment. It certainly limits me in some aspects of RBR.