Look at New Zealand - they lack agricultural subsidies and tax fuel. Assuming monoculture is because of subsidizes isn't based on reality.

Externalities are a /way/ larger issue than just monoculture or even modern pesticides and fertilizer though.

Vs. the genocide if we didn't use artificial nitrogen fertilizers and had to suffer lower yield for a purely "organic" food system (note that organic fertilizers that ultimately source their nitrogen back to waste from conventional agriculture could not be used.)