Why do think we need to leverage fossil fuels to build rockets? My impression was that solar and wind are already out-competing coal in terms of electricity production, and are close to beating natural gas too.
I also don't think fossil fuels are required to build rockets. Hydrogen can easily be generated from electrolysis (although a bit more expensive than from methane), Methane can be produced from plants, and while I haven't heard it tried yet I imagine a fuel similar to RP-1 could also be synthesized from plants, similar to how we make ethanol.
Do you think we won't be able to switch to renewables in time, or is my analysis on how effective renewables are off?