This would not fly in a court. I'm writing this again in the context of German legal system (and most of the continental Europe is similar).
To argue a good faith argument, you need to be paying the debt as long as the service is provided. You need to have strong proof that you have tried your best to cancel. Just claiming that you lost your card or not using the service is not an argument. in fact it would aggravate consequences for you since it can be interpreted as a bad faith argument from your side.
Usually this ends up sending them a letter with registration / delivery feedback (Einschreiben or Einwurf in German postal terminology).
The law tries to strike balance between contract law and consumer protection. You get a limited time to cancel without a reason and consequences. The contract also needs to be "fair" for both parties. For example a contract between you and an ISP cannot force you to pay when you move out and the said ISP cannot provide you an equivalent service. So it needs reasonable cancelation clauses for both sides.
But if they have very shady and cumbersome cancellation policies and procedures, wouldn't you be able to win by good faith?
I assume you could also lie and say that you lost your card and also prove that you didn't use the service anyway.