You’re right, it’s dropped more in the past 20 years than I would have imagined.
If rickshaws, carts, wheel chairs and the like are still around, I’m guessing it wouldn’t take long for someone to take the wheels running parallel and put them serial. Chain drives have been around for a long time so I’d say in 5 years we’d be back on our velocipedes.
Thank you for that. 
I’m doubtful because parallel is obviously stable but serial is (to someone who doesn’t know) obviously not.
The world is chock full of engineers who do know that. In fact, anyone who has seen a spinning top also knows the effect, though I wouldn’t expect a lay person to make the connection to bicycles. An engineer, though… things like gyroscopes are used for stability in so many applications that it’s practically universal knowledge among engineers.
Yeah except that bikes don’t stay upright due to gyroscopic effects. This has been established experimentally since the 70’s.
Uh, if they’re not really careful, any bicycle repairmen working in their home garage might accidentally invent an airplane instead. (It happens.)