Consider the entire economy, though. Let's not assume that ethanol could ever replace fossil fuels at the scale needed for explosive technological growth. the reason pure ethanol is cheap in the modern world is because we have enormous economies of scale producing the necessary feedstocks which rely on trucks and trains and fertilizers, hell, even the energy used to distill the ethanol is typically from fossil fuel.
It's about supply-demand. If, tomorrow, there were no gasoline anymore, the price of ethanol would be astronomical.
Note, though, that we are talking about much smaller population - so you could spend quite a lot of land per capita on growing both ethanol source and fuel.
Current size of humankind is clearly unsustainable in this mode, of course.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.