If European startups end up starting companies in the US, and the US companies can't operate in EU, there will either be a massive vacuum to serve the European market ($15 trillion in estimated GDP), or there will be companies that like profits and want to earn some money by providing products in EU.

We can create a middle ground. When ever information about a EU citizen that get transferred to the US, a similar information about a US citizen get transferred to the EU as hostage in case there is a data violation. A list of IP-addresses accessing usa.gov in return for a list of IP-addresses that accessed europa.eu. Surely a deal can be made that give both sides equal power.