If links are a proxy for quality and relevance, then conceivably, machine intelligence will eventually be able to score a site for quality and relevance without the crutch of looking to other sites for "votes".
Then you should try to get AS MANY LINKS AS POSSIBLE NOW - or start to make your web site really crappy!
Because in my niche, the crappy sites have all the links
. So google is going to look at those sites and say, "These sites have lots of links, therefore they are quality sites, therefor sites that are similar to these sites must be good and will henceforth rank highly in google."
visitor interaction will be the future of "sustained" search rankings. It's track-able and not as easily gamed.
It's only somewhat trackable and it is easily gamed.
Even with google analytics, there are problems tracking how much time someone spent on a page. I believe if someone goes to your page and reads it for five minutes, then picks up the phone and orders something from you, but closes the browser, then it is considered a bounce, and the five minutes they spent on your one page doesn't get added to the time on site.
And not every site had google analytics on it. I am sure that if bing ever had an analytics service, many people would switch just to keep their stats hidden from google.
As for not being gamed... well, people still game adwords with click fraud. Also, if there is software that can go to hundreds of thousands of websites and automatically create forum profiles, directory listings, blog comments, what makes you think that there isn't software out there that will rotate through proxy servers and spoof user behavior?
I am sure I could easily spoof a high bounce rate for my competitors and a low bounce rate / high time-on-site for my own pages.