The geese that lay the golden eggs
As Dan Mitchell (from the CATO institute) once said
The geese that lay the golden eggs can fly across the border.
This is such a nice way of explaining how uncompetitive tax rates can lead to successful people and businesses, who both contribute massively in taxes, to leave the country/state when conditions no longer suit them. For a country to grow and be successful, it has no option but to choose low tax rates, and from a libertarian perspective, a flat tax.