These things go in cycles, though (if I can say it that way). LONG cycles.
The 1920s were a high time for bikes, the 30s and 40s a low time, the 50s were a high time, the 60s pretty low, but Japanese innovation caused an explosion in the 70s that lasted until the 90s.
Who knows what's next or what causes the cycles? Wars, economic conditions, alternatives (after the Model T came out, no one in the USA bought motorcycles for transport any more, just like no one bought any more family sidecars in the UK once the Mini came out, or someone might argue the Minor and the Popular and such), or aging Boomers buying Harleys so they can be Wild Hog types ... all sorts of factors.
I ain't worried, though!