There have been many tether-based designs to bring stuff to LEO. It looks feasible on paper but it's an open question whether it would work in practice. It's certainly a lot of fun to imagine a world with these gigantic tethers.
There are many unsolved challenges:
a) to build it
b) to make it work, 
c) to make it cost-effective
And you can't only consider the costs of mass-to-orbit by starting from 30km altitude. You first need to bring your payload from the ground to that platform. All in all, it looks cool but the starship design is really at the frontier of what is currently possible.
Reaching 4% for SpaceX is not just having 1 killer feature that changes everything, it's rather the accumulation of a very significant number of small improvements regarding materials, strucutre, fuel, engines.