Your second point is key: the fixed fee plans compensate poor writers as much as excellent ones. With that argument, I think publishers could sell a variable fee.

Maybe there's a slightly more middle ground--pay based on the number of peak _simultaneous_ borrows in a period. Just as a library would have to pay for each physical copy lent, is it too Luddite to suggest Amazon do something similar? Not pay for each borrow, but based on the peak simultaneous borrows?