> There is clearly a temperature at which this planet will not support human life

Yes, but that temperature isn't going to be reached by fossil fuels.*

The reduced brain function from the extra CO2 (if we burned all of it) may make us unable to adapt to the higher temperature, however.

* Ironically, unbounded growth of PV to tile all Earth's deserts could also raise the planet's temperature by 4 K or so, and 6 K or so if tiling all non-farm land.

Deserts are huge, this by itself would represent an enormous increase in global electricity supply; but also, current growth trends for PV have been approximately exponential (in the actual maths sense not just "fast") for decades now, so this could happen in as little as 35 years give or take a few (both scenarios are within the same margin for error, because exponential is like that).