As for why there are no younger new riders, their priorities are different. Give them a reason to ride and they might join in. Offer up some useful, sensible commuter type bikes like we had in the 70s/80s. Instead of the pricey driveway ornaments and a lifestyle...
I think you may be on to something, but who knows... I'm not sure anything happened other than the normal cycle of things... recreational motorcycling, recreational boating, recreational aviation and recreational auto racing have all taken a hit... I've participated in all of them, and they all ask similar questions, and usually someone will try to say it is a generational thing -- perhaps in part, but not exclusively... All of these sports require money, in some cases quite a bit (motorcycling often being one of the least expensive), but when the economy takes a hit, one of the first things we quite spending money on is items we purchase with disposable income, boats, planes, bikes, RV, etc... largely because we're not sure how much we'll have... historically it takes at least a generation or more to recover... then there is the thing with the Boomers, and inordinately large generation with relatively large amounts of spare time and money -- most generations won't be so fortunate, so we return to more normal times...
As for those missing the camaraderie of the open road -- not all of us rode with groups... I tried it once or twice, and found that I often rode much farther to get to "the ride" than I did on the ride... not for me, but we all ride for our personal reasons... it'll be what it is...