If I am not mistaken, there was a post on +SEOmoz
about the future of SE ranking factors and the idea was that back-links will be the core ranking factor till the time when whole scheme/protocol of the Web changes and is not going to happen in the near future.
For me there is no way out. Why? Just imagine Google somehow can determine the link is irrelevant (e.g. by scanning keywords on that website) and stops giving value to it while as there will be still value from relevant websites, which can be easily found our by SEOs. What happens next? Will they stop selling/buying links? No! They will be purchasing links from relevant website, website owners will have needed keywords on their website to selling links effectively and the prices will be much higher than now.
Purchasing or not purchasing back-links is rather a philosophical or ethical question, but a SEO one.
There is a way to stop buying links - give no weigh to back-links at all, but off site ranking factors have always been of major importance because it is something (as considered) website owners can't control. Then what will be left - on-site factor and social signals?