You are looking at a very small fraction of the problem. Yes higher water is not an extinction level event.

But with increased CO2 there's also increasing acidification. So the plankton dies, coral dies, mollusks die (their shells erode). The food web at sea dies.... as does the food for a decent fraction of the planet. Acid rain causes more damage, food production becomes tougher... especially on the decreasing number of acres above flood level. Much of the most productive farm land in the world would be inundated with acidic salty water.