edit: nor is there meaningful capital inertia behind synthetic vs. other fertilization.
To not use synthetic fertilizer, today, would increase food costs today. To run out of synthetic fertilizer e.g. 150 years from now would increase food costs an equal or lesser amount. Potentially lesser as a result of increased time to develop better sustainable methods. Integrating cost over the next e.g. 250 years, it will always be cheaper (better) to use the non-sustainable resources at the time sustainable methods are most expensive (today). Externalities, like global warming aside, as they aren't relevant to the example of fertilizer I was replying to.