Gas, including ethylene from oil production, is used heavily in pharma, plastics and fertiliser production. If oil becomes uneconomic to produce, the supply of drugs, explosives, plastic and fertiliser is at risk, at the very least of price shocks.
Peak oil is not really just about untapped stocks, it's about the economics of supply and consumption. We won't "run out" of oil, it will stop being viable to produce it when gasoline ceases to be so economically important.
The feedstock uses of ethylene and lng are justified by the underlying consumption of petrol, diesel and heavy oils. It's profit from waste. If the gasoline isn't used the refineries shut down. If the entire cycle has to convert to fertiliser the economics of fertiliser change, and prices change.
Would you elaborate on what you mean by this? I don't follow.