We can always make polymers and HydroCarbons in general from other sources if we have energy abundance. We literally can just capture the CO2 we emitted from burning fossil and make it plastics.

Of course this does not make sense in a world where we do not have enough energy to even keep datacenters open.

Edit: To clarify, I do not propose burning fossils to capture CO2 and make plastics. I am a Thermo Laws believer.