Even after WW2, there have been advances in non-oil plastics. They haven’t disappeared, they’re all still here… just overwhelmed by the mountain of “get rich quick” scams that are throwaway plastics.
And plastic was not even intended to be throwaway! Over the years it got engineered to sell as many units for as high a profit as possible… and what better way than selling single-use easy-to-break plastic products.
Still, if all oil production stopped right now, we wouldn’t run out of ways to make plastics, just out of ways to produce cheap plastic waste.