You shouldn't say this to people like it's some obvious truth. There are many cases in which this action will land you in trouble due to it not being a legally valid termination of the contract (which of course may be different by country -- it's very common that cancelling requires an actual message to the other party).
One specific example is if your contract has a termination period, which is pretty common, at least in my part of Europe. If you simply stop paying, you are denying the other party N months of revenue (your cancellation period) that you are contractually obliged to pay. You are now defaulting on your payments and will likely pay additional fees.
This makes sense if the contract indeed has a minimum commitment that hasn't been reached.
But if the contract has no minimum term (or it has since passed) and you've made a reasonable effort to attempt to cancel with no success, it'll now be on them to recover the money through legal means which would require them to explain to the court why your cancellation attempt was ignored, demonstrating their bad faith in the process. That's not something they want to do.
The point of my comment was "this is not good general advice". The point of your comment seems to be "it can be good advice in some cases", which makes no sense to me. Obviously it can be good advice in that exact case where it makes sense, but it's not good general advice.
I'd argue it's good enough general advice and would apply to most online subscriptions as they typically have no minimum commitment. The ones with a minimum commitment would be the outliers and would require special treatment.
I agree with your point that you could get into trouble for violating your contract terms. I perhaps should have mentioned specifically about NYTimes which seem to have designed around people blocking the payments to cancel their subscription.
I had to do exactly that with o2 Germany. They continued to charge me after the contract expired. And they even tried to charge for the router that I actually sent back.
This was the whole issue though. I closed my bank account and moved country, and they delayed cancelling it and then chased me up on one month's payments for years - when I had no easy way of making payments in Germany.
In the end I paid it though, it was only 20 euros!
