Nuclear, or if you want space stuff, build a station at the Lagrange point and sling from there? I don't need to be an expert in launch solutions to know that we shouldn't speculate mining the moon till it's well researched and that we have more accessible options here for the forseeable future with solar as my choice of example, so let's not jump the gun on another finite resource?
Do your research before acting like you know what you're writing about. There are so many things to complain about when it comes to capitalism, this is not one of them.
Can a solar sail propel an object to relativistic speeds? The whole point of space travel is to go to other planets at a speed fast enough that the people going there will not be dead or elderly by the time they reach their destination. The only way to do that is by achieving light speed or damn near it. I do not want to board a solar sail vessel bound for Proxima Centauri b (4.22ly) and be dust by the time I get there.
Actually, better than that, I'm not an expert! There are professionals whose jobs are to design those systems and could do better than I putting together a solution. However, more destruction of finite resources when there is easy access to an unlimited resource should be limited to final resorts.
I think corps prefer using finite resources to make customers depend on buying them. Imagine being able to recharge your car by what you produce with your solar panels on your roof: you will spend money only on the car itself and not on the fuel.
I now require people who offer me to try out a solar powered car to provide directions to said car, lest your comment be labeled a sarcastic cock-tease.