If I understand correctly, the issue is not even the citizenship of a clients. The issue is currency. If any two humans in any country wish to transact in the Swiss Franc then that transaction must go through Swiss clearing bank. If they wish to transact in the USD, then it must go through US clearing bank. In both cases client bank must comply with the clearing bank rules. Swiss banks can totally sever interaction with USA, but only if they will also never ever transact in USD.

AFAIK significant banks didn't really shut down over FATCA, some have paid fines or higher tax rates and most of them do the minimum for anyone who matches any of the criteria for persecuting a possible US person, within the limits of the institution's local laws, if any.

From a general government structure POV, I respected a Congress more than the prevalent US views, but I don't respect the US Congress any more.