If you're a US investor, investing in US R&D is easy, you have a good idea of how things work and how to get justice if you're defrauded.

If you want to invest in another country, that's a big change. There's certainly opportunity there, but without knowledge and contacts, it can be very hard to get things done.

One track to investing in foreign R&D is foreign nationals come and work in the US to earn skills, knowledge, and capital, and then they take those earnings and invest them in their country of origin, maybe living here or there.

Yes, I know; but we're talking about what would happen in a hypothetical world where US R&D innovation mostly stops happening, not for lack of money, but for lack of talent; so US investors no longer have any interesting domestic options that are likely to bear any fruit at any multiplier they'd be interested in.

Sure, investors could just park their money in what few dumb domestic options there are. That's the "patriotic" approach, and in less-aggressive markets, you'll see some investors [esp. big institutional investors] building the hedge parts of their portfolios out of these kinds of investments. But when the only domestic options are dumb/boring, any "smart money" investor will either take their money and leave the country for greener pastures, or they'll pick up the skills required to play in foreign markets.