Sure. It's a numeric problem. The proper question isn't "should we build an asteroid shield around the earth?". If it's free, then yeah we should go for it. But

Sure. It's a numeric problem. The proper question isn't "should we build an asteroid shield around the earth?". If it's free, then yeah we should go for it. But if not, then we should first ask ourselves questions like "how much will it cost?", "how likely is it that an asteroid impact will occur?", "how much damage will an impact cause relative to the cost of prevention?", and "how does an asteroid shield relate to alternative preventative measures?". Lomborg did the math and he decided that policy which focuses primarily on emissions reductions is unlikely to do more good than harm. Though with different assumptions of course you wind up with different conclusions.