It's not if you an American.

>Any other country could also open its border to "brains" if they wanted too. The world would probably be better off if smart people (or really any one) could easily migrate to where they can be happiest.

And perhaps we could dispose of the not-so-smart left behind while at it...

Mind: my own theorising, though based on a considerable bit of research.

Monopolies, generally, tend to form around networks, most generally described as flows (material, energy, products, people, information), between nodes, with some cost function. That last isn't just the traditional capital and labour, but a larger set, generally: materials, energy, time, effluent, skill, technical knowledge, O&M, comms protocols (including language choice), access to finance, frictional losses (from hull drag to bribery), and yes, labour and capital.

I'd also add unintended consequenses, though that's a long aside.

Different locations may have favourable or unfavourable standing among various of these factors. Comparative advantage (and disadvantage).

Something like Kirchoff's law of circuit flow may apply, I'm looking into this. There's some prior art, I believe associated with Jules Dupuit.

Edward Glaser, Harvard, makes the interesting observation that cities are an exception to the Anna Karenina principle: major global cities tend to have their own individual focus of competence. This seems particularly the case for ephemeralised goods: television (centering on NYC again), cinema (Hollywood, though, via language frictions, also Bollywood), finance (NYC, though with regional centres in London, Frankfurt, and Tokyo). And of course the San Francisco Bay Area for software, thoughthere are a few other competing centres, notably again NYC.

Glaeser also notes that diverse cities with non-monolithic industries and corporate footprints, tend to maximise diversity -- hecontrasts NYC's garment industry with the company towns of Ford's Detroit and Carnegie's Pittsburgh.

(The bulk material, energy, effluent, and labour requirements of early-20th century auto and steel production are other aspects he fails to dwell on, to his loss.)

But there are also things ccities need to get right: a baseline. Size. Food and water supply. Waste removal, public health, and crime prevention. Transport and tradee. Comms. Quality of life. Fail in these, and aa city wwill decline (though economic failure seems the more apparent proximate cause).

For intellectual and creative activities, opportunity, freedom, removal of barriers & hassles, and lack of oppression, seem key. The mid-20th century transatlantic flow, in both directions, of European Jews west, and American Blacks east, is a particularly inteeresting case: a given location can simultaneously attract and repell.

There are other dynamics: Dutch Disease, Baumal's Cost Disease, Gresham's Law (particularly applicable to brain drain), probably Jevons and Leibig's law.

But getting to your assertion, simply opening borders is far from sufficient.