I too had the same thought after riding the Magic Pie 3, a couple miles without power.
The MP3 is direct drive and mine will spin 3.5 turns before it stops.(just with a spin with my hand).
I recently put a little geared motor, 250w on a bike. It spins and spins and spins. Just like a bike without a motor. No cogging.
However the little geared motor is a doggy gutless thing and for me its really only good for flat hard ground.
I just bought a mac10t geared motor, 500w and it free-wheels good but there is a bit of resistance with it. More magnets may slow it a bit. Still it feels mostly bike like, don't really notice the drag when ridding.
I don't have any seat time with mid drives, but they should feel like a bike with the motor off. Mid drive would have a free-wheel so no cogging.