Yes, corruption is likely a cause of brain drain, I agree with you there, but I have to think that brain drain is a multifaceted issue. Some of these facets are out of the control of the locality entirely.

Some examples:

West Virginia can’t change the fact that there’s little demand for coal mining.

The Midwest can’t change the fact that the manufacturing industry spread out beyond its borders.

Vacation towns in Michigan can’t change the fact that cheap air travel now exists to take people to more desirable beach climates.

New Orleans can’t change its physical location to be less prone to flooding.

I don’t have enough familiarity with Portugal to say what’s going on, but any time I hear high taxes being blamed for widespread issues I get a faint whiff of oversimplified libertarian ideology.