This seems super simple. What is its efficiency?
Sure it spins with magnetism and momentum, but to generate any substantial amount of electricity you will need to use metal windings of a determined length.
This is going to add weight, meaning you need bigger magnets to move the weight and better bearing materials. You also need structural bracing to keep the machine stable for high efficiency. Bracing will add to the energy cost substantially if it is made of metal.
The energy cost to make one of these needs to be overcome within the devices life at bare minimum. If you keep repairs to a minimum and durability to a maximum, maybe?
Id like to see more.
I juat want to say I share your thoughts.