What happens on a rainy holiday weekend and Tesla drivers are lined up at the solar-powered supercharger, especially once cheaper Teslas start to proliferate?”
In a hypothetical future when Teslas start to proliferate, there will be hypothetically more charging stations. Today, Tesla is going gangbusters building new charging stations, it is not like they are going to stop once sales start taking off.
Also, all superchargers are hooked into the grid, so they still provide full power even when their solar panels don't produce.
Honestly, how long do you think the “free charge from a supercharger” incentive will continue as Tesla has to pay more and more for power off the grid as more cars use their services?
The “free solar power” thing is nothing more than marketing hype to impress the rich greenies.