I think if Bing, Yahoo, DuckDuckGo, Blekko, and other search engines become massively more popular, I might see a slight increase or decrease in traffic depending how other search engines rank websites in their Search rankings unless other search engines mainly care about Backlinks from very popular websites like Wikipedia, AOL, Yahoo news, Gawker, Colleges and old media websites like the New York Times, USA Today, CBS, ABC, BBC, etc, or the search bots or crowd sourced members like DMOZ Directory are smart enough to read a website, and know if it is quality or low quality by reading it once.
Re: If other search engines become more popular=more traffic
It is always going to be a hard market out there to compete with other sites no matter which sites you are looking at ranking high. Chances are, when those search engines become more popular, other admins and webmasters will take notice as well. That means that you will most likely be at the same position you are with Google. But, I am always a fan of spreading out your site and getting it listed on as many search engines as you can. It can't hurt.
Re: If other search engines become more popular=more traffic
I think if the other search engines were set up like DMOZ http://www.dmoz.org/ or the Yahoo Directory which uses real humans to add websites, and edit the search engine submissions rather then using computer algorithms/trends, lots of websites might be lowly rank, or not even accepted because the humans who run the search engines will reject a lot of sites because of poor grammar, spammy content, lack of activity, and a bunch of other reasons.
But, the search engine users will probably get better results becuase all the sites which are spammy, and dead/outdated won't be accepted into the search engine, or won't be ranked well.
