The decrease in the number of Swiss banks prior to the UBS-Credit Suisse merger was largely a product of Swiss bank consolidation in the 1990s, not the US shutting them down. UBS and Swiss Bank Corporation were the first and third largest Swiss banks when they merged in 1998. Credit Suisse was the second largest Swiss Bank when they acquired Switzerland's then fourth largest bank, Volksbank in 1993 and they had also acquired Switzerlands oldest bank, Bank Leu just a few years before that.
UBS paid a nearly $800M fine to the US, they didn't collapse because they could pay that. At least a couple of Swiss banks collapsed when they went through actual prosecution, but around 100 Swiss banks paid absolutely enormous fines to avoid prosecution in a political deal between the US and Switzerland with the implication that if they continued helping Americans hide assets they'd be prosecuted for all of it anyway. The fines were actually sized correctly for once, 20% to 50% of the total of assets Americans had hidden in them.
