Think of negative cost energy as paying someone to dispose of something you don't need, like how trash haulers get paid to take stuff away.

If we get to the point where we have a bunch of generation for peak demand plus storage, and still have excess energy in other times, we can and should investigate fixing that excess energy into stable chemical molecules that are useful for other things (whether it's good for a material, for energy storage, for some other consumable like fertilizer, or even simply reversing carbon emissions by fixing carbon into stable non-CO2 compounds).