Content is not the only thing that gives a site value, value can also be found in traffic. The value in traffic can be found by analyzing the number of unique sources or search terms that provide traffic for a website.
If a website is kept alive by only a handful of search terms then it is very vulnerable to search algorithm changes and thus is not likely to survive long term. One easy way to increase the number of search terms that provide traffic is to increase the amount of content you have (another reason why continuing to build unique content is a good idea). The more content you have the more varied it likely is and so the more varied the search terms used to find your site will be. When you're site is found using thousands of different search terms you're not as likely to feel any fallout from a search algorithm change.
You should also look for diversity in incoming links, and this is one area where established sites really have the edge. New sites, especially those published by someone who already has other sites, typically start out with links from directories and usually other sites owned by the same person or company. While this can be enough to give the new site a decent rank, that rank is fragile since it is dependent on links from only a few sources. It is much better than have 1000 links from 1000 places than to have 2000 links from 10 places. Have a more diverse pool of incoming link sources usually means you have a good deal of links from on-topic pages as well, and that's definitely a good thing.
By diversifying your traffic sources as much as possible you lessen the chance that a single event could hurt your site's traffic enough for it to fail. As such the more varied the sources that provide traffic the more long term value that traffic has.
I know many of you are likely thinking that you should focus more on sites with long term value instead of making sites that can hope for a temporary existence only. Well that simply is not true. Just because a site has no long term value does not mean it is a waste of time to make. Sites like datafeed driven sites can still be very profitable in the short term. You simply need to be aware that such sites are not likely to produce consistently in the long term and so you shouldn't do anything like buy a house based on that income alone.