The physics of the situation cares not at all what you believe.
A few degrees warmer means that feedbacks kick in that will make it more than just a few degrees warmer. And many species, already under stress from our population, will find their niche gone. So even as the human population undergoes a rapid diminishment, the other species will be doing the same, many to the point of extinction.
you assume that the human population will rapidly diminish because the temp went up a few degrees?
That's a big assumption.
No bigger than assuming it won't! The problem isn't the few degrees, but the amplification of heat as higher temps lead to more vapor in the atmosphere and then, as we're told every day because water is the major GHG, MORE HEAT!!!