They miss the point when they write "Who's to say that App.net will always cost $50 per year? What if, a year from now, it needs a lot more to keep the service going. App.net has incentives to figure out ways to raise the price to bring in more money."

If it turns out that the price charged is insufficient to maintain the service, I'd expect them to raise the price. The alternative is for the service to not exist, taking as an axiom an ad-free service.