In an age where card payments are ubiquitous, being suddenly cut off from VISA/MasterCard networks can severely disrupt a country's economy. Especially if it heavily relies on tourism.
The EU prevents sellers to surcharge depending on the type of card used (PSD2 Directive (EU) 2015/2366), Art. 62(4)). It in effect opened the door to a Visa/Mastercard duopoly, as no local competitor could emerge and compete on price.
When it comes to tourism, this problem will always happen if the tourists are coming from the side that's cutting off the other. Without an interface between European and American payment systems, Americans won't be able to pay in Europe.
It's way more severe than that since Americans are not the only ones relying on VISA/MasterCard for payments abroad. The presence of other payment systems would make it easier for any non-USA country to still do payments.
