The largest breakthrough by spaceX is the realization that fuel not the largest cost of getting to space. They do several things which dramatically reduce cost at the cost of using more fuel but it's a net win.
This is exactly the strategy outlined in the real-engineering fiction book The Rocket Company. The vehicles in that book also use friction-stir welding, though they are all aluminum. In that book, some billionaires get together to fund a fully reusable TSTO rocket operable from the field.
