The argument’s kind of absurd; at a certain price point, oil from the shale formations in the West, and coal-derived oil will be competitive with petroleum, so there’s little real likelihood that there won’t be enough oil or oil-like feedstocks for chemical processes in the foreseeable future.
Ethanol is produced from the kernels. There’s research going into ‘cellulosic ethanol’, which would extract it from stalks and leaves, but no cellulosic ethanol program has escaped research stage into commercial production.
There are other posts above about whether it is or isn’t net energy negative. My point is that even if it were hugely net energy negative to grow plants for biofuel/plastic production, that would still work for the OP. We’re positing only oil running out, not the non-existance of easily creatable electric tractors powered by coal, wind, propane, etc. IE, the process is at worst energy net negative, there is no reason it would be oil/plastic/whatever net negative.