If I was a motivated SEO booster, I would just pay a room full of people somewhere in the world to "downvote" all of my competitors search engine results. The problem still exists. As long as there is an incentive to be gained from SEO spammers, they will find a way to influence search results. This transcends Google or DDG or any specific search engine susceptible to SEO abuse.

There's a great talk by Anil Dash that I like titled "The Web We Lost" that centers around on this topic - https://anildash.com/2012/12/13/the_web_we_lost/