It's so bad, there are even companies that handle cancelations by mail for you. You enter your contract details on their website, they print it and send it by registered mail.
Why don't people just stop payment? I remember when an insurance company tried to not let me cancel. I copied some boilerplate termination letter from the internet, and blocked payments from my card. They cancelled very quickly. Then again this is in the USA, so Germany may work differently.
In Germany if you have a contract which binds you for a certain amount of time, blocking the payment will just result in letters, extra fees and eventually collection agencies and a state bailiff. This will not just put a stain on your credit score but might also get you into legal trouble.
There are multiple reasons. You often don’t subscribe by credit card but by bank account (you authorize them to pull money from your account). This means you cannot cancel, only the company can cancel. If you want to cancel you must hire a 150€/h lawyer to take them to court and fight a lengthy lawsuit that you will lose. If you did subscribe by credit card and block it, they will hunt you down to the last cent. They have the lawyers and debt collectors while you don’t.
Sure, debt doesn't just go away. But there's no loan, so why are you talking about debt? The grandparent comment is talking about online subscriptions. If you stop paying your your subscription, then they'll terminate service. Likewise if you stop paying your insurance premium you won't be able to file any claims. But unpaid subscriptions don't magically become debt.
If you don't pay your electricity or water bill then that's a different story because utilities are not allowed to just stop service for things people's lives depend on. But utilities are more heavily regulated than normal subscriptions. For the most part, if a company tries to keep you from terminating a subscription and you say, "well, I'm not paying anymore. Do you still want to keep providing the service for free?" it's an effective way to get them to terminate service.
So if you block your Netflix payments you really think there going to send debt collectors? No, they'd lose more money on hiring a collector than they'd gain back from recovering one month's subscription.