If we get to a grid that's ~100% renewable, we'll likely have oversized it a massive amount, like 50%-200% more energy could be produced than is needed at the time of productions, to deal with seasonal and weather variability. Let's call all that energy free, because it has zero marginal cost and would be curtailed otherwise. Who pays for the capital on the equipment to pull out CO2 from the atmosphere, and the disposal of the resulting tons of carbon?