See my sibling answer next to yours for an explanation of why the oceans were not then acidic despite the higher atmospheric CO2 levels. If you wait a few thousand years, ours won't be either. But shellfish who depend on their calcium bicarbonate shells not dissolving can't afford to wait a few thousand years while bathed in a mild acid...

I agree with your main point, but I'm pretty sure that evolution and adaptation work for humans too and there are an awful lot of human beings.

None of this is to discount the potentially severe (unknown, scary) risks, but I don't think human extinction is really in play due to a warming climate. The most serious risk I'm aware of is that high CO2 content in the atmosphere will interfere with human respiration. Evolution will handle this, just like it handled human beings living in the Himalayas.

How is this not fear-mongering? Most people will be fine with a 5deg C global increase.