The article makes the point that even if the rich don't leave, or avoid their taxes in some other way, high taxes on the rich disincentivize creating wealth. That's likely to have a drag on the entire economy.
People may not like the fact that much of our material, technological, and medical progress over the past century has been a result of people trying to get rich, but it's still a fact. If getting rich becomes less attractive due to high taxes, fewer people will try. Everyone will suffer if the economy slows, and the non-rich will suffer more than the rich will.