My thoughts:
1. Boomers, of which I am at the tail-end, made up most of the sales. When I was in my 20's many of my friends had, or wanted, a motorcycle. Those are the same people riding today. Riding brought a sense of freedom that we wanted. The dangers of riding were part of it, we accepted the risk. People have always said that riding is too dangerous.
2. Too many of the offspring of Boomers were babied and, many, not allowed to experience risk. Jump homemade ramps on your 10 speed bike, No way, you could get hurt! Go hunting squirrel with your BB gun, No way, you could put your eye out kid or don't kill those pretty animals or we have plenty of meat in the fridge, there is no need to kill a squirrel for food! A 10 year old with a .22 cal rifle, how irresponsible!
3. This is perpetuated by those of us already riding: "That bike is not big enough!" When I started riding, Gold Wings had about the biggest engines because they were big touring bikes. A 750 was considered a larger bike and people were happy with it. Today, a 750 is "not big enough" compared to the gargantuan engines there are today. The "not big enough" mentality has brought much larger, higher priced, bikes that the young "entry level" rider cannot afford. My son's first bike was a Kawasaki Vulcan S, 650 that was $5400. I had friends tell me, "That's not big enough." It was plenty big. It was big enough for those nay sayers 30 years ago, what's the problem today?
There are many reasons for slumping sales. I read that the adventure bike is the most popular right now, can't remember where I read that to cite it. Don't remember who posted it earlier, but, the younger generations are stay inside, play the video games, text your friends, and watch Netflix types. Ride a motorcycle? How can I text my friends?