I didn't say it'd kill all complex life, I said complex life would be greatly impacted.

For example ocean acidification is tempered by reacting with build ups of calcium which is the building blocks of many things in the ocean. Shelled critters and corals immediately come to mind as examples of directly impacted complex life.

As the corals die and can no longer form due to acidification that whole ecosystem collapses.

The stuff that eats the phytoplankton (sensitive to ocean acidification and heat) no longer can eat it due to it dying along with the other little micro organisms, also suffers from ecological collapse.

A big issue that impacts complex life is how quickly it can adapt to the changes in their ecosystem and if they can find new places to go or new things to eat.

For example E. Coli: it has quick generations so it can adapt really quickly. This experiment has been going since the late 80s and the E. Coli has gone through over 70,000 generations and they've seen a lot of changes. If you went back that many human generations it would take you back before modern homo sapiens.