let's take the case of a client refusing to pay for incidentals for a car-rental example from strip's documentation.
what if the customer's bank requests the client's authentication, but the client refuses to do so... (refuses to pay)
before, we could directly bill the client even if he refuses to pay. but with 3DS I can't see a way to do that
In that instance you'd sadly probably need to get the law involved, as after September 14th there will be no way to charge a credit card in the EU without 3DS.