Oil will only be in demand as a chemical feedstock as long as it's economically competitive with the alternatives. There's a substitute for virtually every petrochemical process if oil becomes scare (or expensive) enough.

Substitution is highly impractical in the short term but in a conversations of decades/centuries it's significant. Venezuela's reserves alone could run the world's petrochemicals for 60 years (Gemini) so it's a realistic perspective. Together with other proven reserves we could be okay for centuries.

Recycling is sometimes an option too.