I had a discussion with a friend yesterday who has owned over 100 motorcycles age 73. His current passion are BMW R90S models, Norton's, and Ducati's. He got out of his Moto Guzzi phase a few years ago due to poor dealer support.
He made what I thought was an interesting statement. "With current technology, a 400cc-500cc bike can provide pretty much all the performance most people need. They are even capable of fast highway speeds for long periods of time. It is interesting to see more manufacturers focusing on that size range."
I remember thinking the same thing a dozen years ago about 650cc bikes.
Advancing technology, advancing age, smaller lighter bikes, there may be a connection.....