DuckDuckGo can't even answer simple queries like "how old is Donald Trump", "president of France", "how tall is the Eiffel Tower", "distance from earth to Moon", and "distance from earth to sun"; it forces you to search its results for the answer. For all of queries, Google and Bing give you the answer front and center without making you click links or read the result snippets. Bing even has an interactive solar system viewer when you search for that last query.
These days search engines are supposed to be able to surface and present information from sources without user intervention. DuckDuckGo cannot do this (yet).
Neither did Google back in the early 2000s. But people still used it and loved it. I would say DDG2019=Google2007 or better in terms of search quality.
Instead of thinking that I must use the best search engine, regardless of the ethics of its business model; think that I will use the good enough search engine from among those with ethical business models.
Edit: I would additionally like to add that Google's search quality peaked early to mid 2010s, so DDG is not far from peak known search engine quality.
These days search engines are supposed to be able to surface and present information from sources without user intervention. DuckDuckGo cannot do this (yet).