Maybe there would be less money in it, but it is possible to do advertising completely on the backend as well. You send the advertiser some targeting info, e.g.: what does the article(s) on the page talk about, what is your site about, what is an estimated profile of your readers, and the ad network gives you the ad images and text in some nice format.

Yes, it does require confidence that the publisher will actually show your ad, but it was the same thing for the journals. On the web you can still track outgoing links and have the referer, so you can know the actual impact of your ads anyways.