I mainly agree, but it could be substituted. Various biomolecules are being investigated as a replacement substrate for established (petro)chemical processes. Part of the issue is, that you need to defunctionalise the chemicals which is the opposite of what petrochemistry currently does (which is adding functional groups as needed, not removing them).

This research, however, is stifled by the cheap Price of oil. I know an anecdote of Nivea pulling their funding into a similar project because the price ber barrel recently fell. The project was supposed to last around 5 years.

I argue that if oil wasn't as cheap, ecological alternatives to plastic would have a chance or would be considered at all.