My bet: Moon based vehicle and propellant manufacturing. It's just so much easier to get stuff off of the moon that if you were going to build anything of any real size in space, the moon is where I'd get the bulk raw material given the choice.
I think Starship is like entering the wide-body jet age of aircraft. A lot of fine-tuning and efficiency gains from now until it's ubiquitous and cheap. But getting from that to another 10x gain is much harder, similar to aircraft where we haven't seen a "10x" jump since wide-body jets.