If the world would just consist of the USA, Canada and Europe I would agree, we could handle it just fine. It will be expensive, especially in infrastructure costs due to sea level rise. Transitioning our forests to species adapted to higher temperatures will be a pain, same for changes in our agriculture, but we would be fine.

The problem are the nations with megacities at the coast, but no money to build massive sea walls. Or the nations that are already at the edge of survivable (wet-bulb) temperatures in the summer, but little AC coverage (and often too little electricity to meaningfully increase AC rollout, even if they had the money). Or the nations that have to compete with their neighbors over water sources for drinking and agriculture. Those nations will all be in significant trouble. Which will probably mean migrant waves that will be on an entirely different scale compared to what we are used to, and quite possibly beyond what we can handle. But at the same time, both it's hard to stop desperate people from moving (morally and physically)

Oh, so the mitigations are barely starting to work and we should increase our efforts since the net effect is working (no way have we reached full diminishing marginal returns).

Cool. Agreed, doomerism is the wrong approach. But there is still massive cause for concern, and we should proceed forward.