I'm one of these European users. An American shop only offering payments via Libra would end up the same fate as American shops that don't offer PayPal or another convenient payment method do right now. Why would I even remotely want this? These crypto threads make it out like it's the second coming, we have a more or less functioning financial system and even if it's broken I would not trust Facebook of all companies to offer up a better, global, alternative.

The forming of a financial underworld in the Eurozone is pretty doubtful in my mind. Sure, enforcement varies country by country but if this would get a big enough problem we also have international efforts in the EU to get rid of it. Worst case somebody gets a data dump in lieu of Swiss tax evasion dumps on CDs a few years in and the laws against it get even stricter.