You can't just build a firewall for data, especially as users will actually willingly export data. You look at the GDPR from a business side only and miss that it is about personal data and how that data has become a commodity that is being traded, mishandled and often abused.

So far none of the US-based services that I use has shut down or blocked me just because I'm under the protection of the GDPR. Those websites we see blocking users have either no interest in the European market (fair enough) or are indeed using shady practices.