Making improvements != economy of scale. Digging 10 tunnels in parallel using 10 machines and 10x the people I bet won't save you much in the grand scheme of th

Making improvements != economy of scale. Digging 10 tunnels in parallel using 10 machines and 10x the people I bet won't save you much in the grand scheme of things. And the reason why they "throw away machines" is because most of the time those are custom built based on the project and the wear and tear is substantial.