Over tax the super rich and they'll just go elsewhere.
Over tax corporations and they'll just go elsewhere.
It may be true that individuals will go abroad, although I doubt that it's aywhere near the numbers that we're led to believe.
Corporations will not however. They will use whatever tricks their acountants can devise to pay less tax, they will offshore themselves, double dutch etc. But at the end of the day even if the government manages to plug all the loopholes, there is still profit to be made so they will stay and make that profit, even if higher taxation has reduced it by 10%.