I'm more pessimistic. People will come up with something to use up all easily available fossil fuels no matter what population size. E.g. via space tourism, the

I'm more pessimistic. People will come up with something to use up all easily available fossil fuels no matter what population size. E.g. via space tourism, the moon being the new Mount Everest. This can easily be seen by how much more CO2 each US-Amercian emits compared to every other person on earth. So your Thanos solution wouldn't even work short term. The effect also doesn't wear off until you take the CO2 out of the atomsphere and bury/solidify it, which doesn't happen organically within reasonable (for humans) time frames.