Your explanation is simple malthusianism and doesn't address my argument. People have burned coal for millenia. It was not until Capitalism that it become systematically exploited along with many other things to produce unprecedented abundance. Capitalism is the most efficient system at exploiting resources in history. That is why the population boomed, the necessity for growing the workforce and markets enabled by new innovation and exploitation.

but then the answer to the question of "why more slaves today?" would just be "because we haven't built socialism yet"

I still don't know what kind of point you're trying to make with this. No one would explain anything by the lack of something to bring it to an end.