I expect in a post-fossil age we'll get organic chemical feedstocks from biomass, and non-biomass solar will help with that by providing hydrogen for hydrodeoxygenation. That will roughly double the hydrocarbons one can get from a given quantity of carbohydrate. Biomass here will also include waste stream organic matter.

Of course they are. We aren't going to be able to take hydrocarbons out of the ground forever.

You won't burn them in your truck, though. That's an almost certainty. But whatever use they still get when we end transitioning to solar will be met by synthetic hydrocarbons, there's no point on keeping the entire oil production and distribution industries when you can just make a bit of it near the point of use.

> Solar made hydrocarbons are never going to be economical.

"Predictions are hard, especially about the future".