We don't need an alternative to petrochemicals for rare use cases we decide are worth the cost-benefit ratio. I expect there will be jerrycans of synthesized gasoline on the McKendree cylinders riding fusion engines to other stars. For the odd use case where you need a stable high density fuel with minimal weight-costs on the engine, and you can tolerate the atmospheric effects it will make sense to use it so we will.

Fossil fuel is hopefully going away, but we will have petroleum in the toolbox forever and we can still use it when it makes sense to.