I'm familiar with a few falsehoods as to why we shouldn't tax the rich. "They are the job creators." "Cutting taxes on the rich stimulates economic growth; raising taxes retards growth." "It's class warfare."
But this is a new one: "They'll just move to a different country."
If and when taxes are raised on the rich, sure the uber rich can move, some of them. So, let the traitors move. But just shrugging and saying "well, we only can tax the poor and the middle class" is a falsehood. Not all the uber rich are traitors and not all will move.
The real reason we don't tax the rich is because we only elect the rich to make our laws. And, over the last 3 decades, they've finally gotten their message straight that allows them to act purely in their own self-interest while appearing to act in the nation's self-interest.