Because there's more to success than simply the amount of people who click on your ads. They say 20% of their readers block ads, I would say that it's fair to assume that of those 20% maybe 5% would be willing to pay over $50 a year for reading their content or whitelist them, probably a smaller percentage of users will simply update their adblocking mechanism to keep browsing unbothered. Everyone else of those will simply stop visiting them. That's a possible 15% traffic drop.
It may not be traffic that gave them money directly like you pointed out but it's traffic from a fairly active crowd in social media, it's the people who link articles to them on places like our own HN, Twitter, /. or any number of other places. This effect leads to a drop in number of visitors who see ads which is a revenue drop.
I do like the subscription model (I'd like it better if it was a Freemium subscription model but alas it isn't), but I agree with the OP in believing that they'll backpedal this change before the end of the year.
