Gigablast, Mojeek, and Yandex (already a source of DDG results in Russia I believe) all have their own search indexes and APIs to access them.

For a completely independent search engine, Gigabalst is surprisingly good in terms of index and relevance. Not as good as Google or Bing, but I think it is a one-man operation. Mojeek also just received an investment to build out their index.

At Runnaroo, I partially use Google as the backend, but I have done a good amount of testing on the above options in case Google or Bing stop being options.

Just as important as the search results, Bing provides the monetization for DDG through their ad network, so DDG is dependent on Bing for two critical parts of the business. I don't think Bing will ever kick them off because it must be very profitable for them and they need to do everything they can to get marketshare against Google.