I believe that your reasoning misses one crucial detail: we do not exactly know how to climate control a whole planet, we have almost no experience with that.

I believe that your reasoning misses one crucial detail: we do not exactly know how to climate control a whole planet, we have almost no experience with that. Attempts to terraform Mars allow humans to try some things and to see what happens. It will allow to make our models of climate more precise. For now our understanding of a climate based on observational studies. Not a controlled experiment. So when we start to do something having some climatic goals in mind, we'll start to learn a lot. And it would be better for Earth if we started with Mars. Though we have no time, but it doesn't seem a sufficient reason not to try get at least some experience from terraforming Mars. It may reduce risks.