You're making the mistake of assuming that "something can't happen" if it is inconvenient for the business. The convenience of the business is irrelevant - it must operate within the law.
And yes, GDPR prevents data from leaving the EU zone if there is then a possibility that GDPR could be violated. That's the crux of the recent court cases in Austria and France. You may not collect GDPR protected data if as a consequence of that collection there is a reasonable prospect GDPR will eventually be violated by ANYONE.
For your example case, all initial data processing would have to physically occur within Europe, performed by a subsidiary not subject to US law, and only after they had reduced it to aggregate data that could not be reverse engineered to get GDPR protected data would they be permitted to export it to America.
And yes, GDPR prevents data from leaving the EU zone if there is then a possibility that GDPR could be violated. That's the crux of the recent court cases in Austria and France. You may not collect GDPR protected data if as a consequence of that collection there is a reasonable prospect GDPR will eventually be violated by ANYONE.
For your example case, all initial data processing would have to physically occur within Europe, performed by a subsidiary not subject to US law, and only after they had reduced it to aggregate data that could not be reverse engineered to get GDPR protected data would they be permitted to export it to America.