I did that experiment a while back. I think (but it's hard to tell) that I perceived the quality falling for me in that case.

Much of the problem with Google is that it's trying to make a guess about what I /really/ want, when what I really want is to find sites that contain certain words or phrases. Tracking me is irrelevant to that task, and if my online behavior affects the search results I get, that can't improve things.

I don't want a search engine to find me answers. I want it to search for web pages.

I like DDG because it (mostly) does what I want a search engine to do.

I'm not really bashing Google here, it's just that Google is a tool that accomplishes something different than what I need accomplished.

The one place where I think tracking helps (though it's just a gut feeling - I don't have evidence) is when disambiguating search terms that can be interpreted different ways. Google knows that the tech-related interpretation is probably the one I'm looking for, while DDG doesn't.

I do a better job of disambiguating than Google can do, especially if Google is paying attention to my internet or search history to do it.

If I'm searching for something, it's very likely to be something a bit unusual for me to search for. If it's in line with what I need regularly, I don't search -- I go to my bookmark library.

It's definitely the case - search results in private tabs are less relevant for me.

I think the extent of this depends on 1) how long you've been using their search and other products, and 2) how much your interests diverge from the "average".