Right, but from the point of view of the advertiser, the user would be "Buying something" - access to the contents of the article, which is in principle being paid for by the advertising. This is the model that is failing - and why so much effort seems to be put into (mainly on videos) preventing you accessing the content without viewing the ads.
As for directly paying, this doesn't seem to be an option - see, the whole twitter thing, and the fact that having a paywall effectively prevents developing new readers (unless you are large/well known enough to avoid this problem)
Having a paywall doesn't prevent developing new readers: even if you charge for access, you always have the option of providing a sampling of your current or older content for free. For example, the NY Times has a paywall but lets people read a certain amount of free content before they have to subscribe.
As for directly paying, this doesn't seem to be an option - see, the whole twitter thing, and the fact that having a paywall effectively prevents developing new readers (unless you are large/well known enough to avoid this problem)