Does your bank need to maintain a good relationship with European governments? If not, does it need to remain connected to banks that do?
Using the banks to cut off commerce across borders is an enforcement action for what countries agree are crimes - terrorism, money laundering, fraud, etc. It takes a lot of political willpower and negotiation to use that tool. I sincerely doubt it would be used against American websites that choose to not serve the EU.
And they got cut off from PayPal, and the credit card networks for quite a while.
If the US can use it for selling a product in a store that’s entirely legal to sell under EU law, then the EU can also use the same rules for GDPR.