We've seen this all happen before.
Back in 1999, I co-founded allakhazam.com A gaming database site which was specific to one game, and eventually grew to cover others. We witnessed the complete collapse of the online ad industry in 2000/2001, and as a result we worked very hard at establishing alternative monetization strategies. It worked, it worked very well, and in 2006 we sold the company doing very well as a result.
I then spent 3 years arguing against focusing on an Ad-focused revenue model, and renewed focus on the user experience.
Ad focused revenue models on websites are lazy, and very very broken. With a couple minor exceptions, most people are not swayed by the presentation of random imagery on the sides of the page they're trying to look at. Tech savy users either block the ads in the browser or they are just used to the ads and block them out internally. Most sites use 3rd party ad networks to sell the ads they're going to display, and as a result we get useless context-free ads displaying at the wrong time in a users day. Ads for Hyundai cars are pretty useless to someone reading a video game review for example.
Furthermore, having an ad-focused revenue model means that your customers are not your users. Your customers are now the ad networks or your directly sold advertisement. Your users are just a means to getting your customers to pay you more, and as a result the users often find that their user experience degrades. From articles which take multiple clicks across many pages, to invasive and irritating advertising, to vending of the users browsing habits, negatively impacting the user experience results in dollars for the operator.
Switching to a subscription model allows you to focus on developing content for the users as your customers. You no longer need to have the dichotomy of negatively impacting your users for money. Now you want to please them with a positive user experience, good compelling features and content. Good content is hard though. There are 10001 bloggers out there willing to write content for nearly nothing to free. So you can't assume that putting up a couple articles a week is going to be enough reasoning to get people to subscribe. Also many sites over value the value of a subscription. Make it small and I might be interested ($3/month), if you're going to charge me $20/month you're probably not going to get my money at all.
Subscription revenue is rewarding though. We had 50,000 subscribers paying us $3/month. We strove to constantly add features to the subscribed users. Ad revenue wasn't even a shade of that, despite doing over (at that time) 5 million uniques / day.