True, I guess there's nothing saying you can't have the rotational base of the vertical mill start about 15 feet up and extend up 20 feet. Even if you wanted to maintain the ease of ground-based generators and such, all that means is you need a driveshaft to extend from the mill's base to the ground. Since there is no gearmesh, this would be more efficient than prop mills which would need a 90 degree gearmesh in order to spin a driveshaft.

For that matter, the prop mill loses twice. A vertical mill that extends from 15 to 35 feet would need a straight, 15 foot driveshaft. A prop mill with blades that extend from 15 to 35 feet would need to have it's hub located 25 feet up so it's blades extend up to 35 feet and down to 15 feet. It would need a short driveshaft turning meshed gears (a maintenance item and source of weight and mechanical loss) which rotates that force 90 degrees downward to spin a 25 foot driveshaft. Assuming the vertical mill works at least as well as a prop mill, I can only imagine it would be cheaper and easier/safer to operate. Those are big advantages even if it yielded no more power at all.