This is at least partly similar to the problem the “phone company” has.
More and more people, myself included, don’t have a landline anymore. Yet the cost to maintain the infrastructure for those still on the system doesn’t drop much at all when another person disconnects and drops revenue for the company.
At some point, it will become uneconomical to continue to maintain the landline infrastructure, at which point it will fall apart or will have to be subsidized.
In the solar power case, the utilities must still maintain sufficient reserve power generation capactity to handle the highest possible load, but their revenues to pay for that capacity drop everytime somebody goes solar. That means their cost to generate the power they do sell goes up per kwh.
Close, but it’s worse.
A more accurate analogy would be if the landline and mobile phone user got a reduction in his landline cost whenever he used his mobile phone.