Millennial do ride motorcycles - the US motorcycle market is actually booming. What they don’t ride is big, slow heavy clumsy antiques or big slow, impractical and plain old inherently unsafe by design choppers. Or $35,000 touring rigs poorly assembled by union workers that are worse at literally everything than a $20K touring bike from Europe or Japan.
I don’t know if I’d call the US motorcycle market booming. Sales are still roughly half of what they were ten years or so ago.
As for the slow heavy clumsy antiques, Harley still outsells the next closest competitor (Honda) two to one in the US market.
The median age of a new motorcycle buyer is 47. Harley, Ducati, Honda and BMW are all trying to court the younger crowd with new 300-500cc models. Presumably, these would be good for another rising demographic: females. Harley already has 60% of their sales incidentally.
Harley is even going for the high tech route with an electric bike. I don’t have any particular comment on that. Not a bike for me.
For the record, I have three Hondas (RC51, CR250R & XR650R) and one Harley (FXSTC). Love them all.