I think throttle-only would make them illegal in most EU countries (or, more specifically, they wouldn't be in the same category as bikes anymore); but also, is there really anything implicit in a mid-drive engine that prevents throttle-only?
Interesting point about the chain. What causes more strain, specifically? Essentially, if a certain power is required to go at a certain speed, what makes it different if it's muscle only, or mixed muscle/engine?
It’s not that a mid-drive motor adds more wear, as you note it should be about equal to a bike with no motor. But the hub motor reduces wear, as you are applying less force to the drive train to achieve the same moving power, since the motor applies force at the wheel directly.
Interesting point about the chain. What causes more strain, specifically? Essentially, if a certain power is required to go at a certain speed, what makes it different if it's muscle only, or mixed muscle/engine?