Traffic is so easy to cheat at, and its hard to police people who abuse the system. One of my websites currently has about 5,000 unique viewers per day, but I could up that to 50,000 per day without spending any money. It wouldn't take much time. Traffic #'s are way too easy to cheat on, and I think they provide the WORST metric to work with.
Real traffic could be good. But know the real traffic is not possible. Like the PR it could give an approximative idea of the power of a site
the pagerank is no longer linked to the traffic of a website... so it will not be easy to replace pagerank...