Steyns point was that there's no point in Islamists destroying any more skyscrapers in cities they're going to take over anyway.
You are assuming a rationality on the part of radical Islamics that I don't believe can be demonstrated. At the root of their culture is an envy-driven hatred of all the West has created. At a minimum, they are driven to destroy some part of the West to drive home the point of their "superiority". The more interesting question is how the emerging configuration of Islam-Russia-China will work together to bring the downfall of the West. I know that both Russia and China have their own Muslim problems, but the temptation to cooperate to bring the West to its knees will be too tempting. I hope and pray our children and grandchildren will be ready for what is coming in the next 20 to 30 years.