among those things, they had no way of supporting 8 billion people with 3% of the population working on farms. Or intercontinental travel measured in hours. Things like that.

Correct, and then new technologies where discovered and developed that allowed those things to exist.

JFC these countries aren't saying "let's ban all fossil fuels immediately and see what happens!" They are working on new methods to do things that don't require fossil fuels.

A super simple example for you: before fossil fuels there were no cars, then there were cars the required gasoline, now there are cars that can run off electricity, and we can get electricity from wind and Solar power.
A society in which all cars run off of electricity instead of gasoline would indeed look different from the society we have today, but it's not impossible and does not require the majority of the population working on farms again.