I think the drawback to your argument is that you're not considering all scenarios. One "worst-case" I can think of is that we charge ahead with measures to combat climate change, harming big parts of the economy in the process, and wind up with the famine, drought, and flooding anyway because we didn't charge ahead with the right measures.
It's extremely important to determine whether the global warming is caused by us or not, and whether or not we can alter global warming whether we caused it or not. If it's mostly natural (and we know the earth has gone through much warmer periods long before we showed up) then the measures we need to take involve protecting our coastal cities and our food and energy supplies. If it's mostly caused by us, then we need to stop doing what's causing it, and probably need to protect our coastal cities and food and energy supplies anyway.
Why does it matter if we caused anything? If the Earth is going to get too hot or too cold for our survival, we should try to stop it. Maybe we haven't affected the climate yet, but if we try we probably can.
It matters because it changes how we would try to stop it. If it's caused by something we're doing, we could just change what we're doing. If it's natural, then we need to find a way to alter nature. That gets into terraforming, which we really don't know how to do, and it'd probably be safer to to learn to live with the climate change rather than trying to stop it.
It's extremely important to determine whether the global warming is caused by us or not, and whether or not we can alter global warming whether we caused it or not. If it's mostly natural (and we know the earth has gone through much warmer periods long before we showed up) then the measures we need to take involve protecting our coastal cities and our food and energy supplies. If it's mostly caused by us, then we need to stop doing what's causing it, and probably need to protect our coastal cities and food and energy supplies anyway.