That’s not brain drain. Brain drain is when high qualified people leave their country, mostly because of the lack of infrastructures costing them opportunities for studying or working in their respective field.

What you’re talking about is capital flight. This is an issue that is systematically raised as a counter-argument by liberals in debates on taxation. The problem is that it is seriously overestimated:

• Leaving a country is a lot more complicated than it sounds: you lose your family, your friends, your culture, your habits. Many millionaires who leave their country end up coming back after a few years.
• You can’t relocate your real estate investments.
• Going abroad doesn’t exempt you from paying taxes (especially exit taxes).
• A country that wishes to do so can prohibit the relocation of a profitable company, or even nationalize it.
• Many rich people who threaten to leave if taxes are raised end up doing the math: if there’s a profitable business, they’ll stay. And in a country that finances its infrastructure soundly and has a good distribution of wealth, there’s profitable business to be had.