The two you wouldn’t advocate for have their own crawlers and index. The remainders which you are advocating for, don’t have the ability to not pass on the result manipulation from Google, Bing or Yandex.

At best they serve as anonymisers, but Ecosia’s (non-profit) business model is telling Google what you search for, and DDG is beholden to USA laws, which means for all practical consideration they are a front for the NSA.