Most polymers are made from really simple monomers. Just snip the "poly" from common plastics' name, look up the molecule, and what can be used as feedstock to

Most polymers are made from really simple monomers. Just snip the "poly" from common plastics' name, look up the molecule, and what can be used as feedstock to produce it. Turning heavier carbon compounds into such lighter molecules (eg. through "cracking") is a staple of oil refineries & chemical industry. So yeah, when gasoline goes out of fashion we'll still be able to use all of oil that's pumped up for those other uses like plastics. And thus less oil will be pumped. But alternatives come online too. Bio-based plastics are a thing, many paints are water based these days (even for exterior woodwork), fossil based solvents are being replaced with more benign chemicals in many household products, etc.