For starters, about half of all people alive today are thanks to air nitrogen fixation. The Haber-Bosch process is driven by natural gas, methane. Food is produced with diesel and fossil energy in general. Up to 10 fossil energy calories for one food calory. The longterm carrying capacity of the planetary ecosystem post-fossil is less than a billion, about 100 million.

So, suddenly stopping consuming fossil fuels will cause lots of people to die, in unpleasant ways.