I think it will happen much much earlier. Next AOL Time Warner et al. will enter the "market". Things will detoriate quickly from then: all these sites will write about themselves to increase advertising revenues. Users will run away and find different means of finding "important stuff" (maybe by joining dc.com or sites such as craigslist, freebay or whatever).
I give this system of intransparent ratings two years. It has been hyped too much. Two years, because there are lot of internet users that haven't seen the hysteria behind this kind of sites.
We have to find a better way to promote things that are important to us; a reliable, untainted way. I do not know of a good way...