But how good can a search engine be if it does not track its humans, and therefore cannot use browsing / search history as contextual clues for query ranking / semantics ?
It might be very good. But can such a system compete with one that does use search history as context?
There used to be a concept of google-fu, where writing search queries was a skill in itself. I got quite good at finding what's needed. Then, over time, search results became less and less useful. I noticed that search results seemed to be weighed more on my history than the query written. I'd get results with words synonymous to web page titles I'd recently visited, but otherwise no connection to my search. To get decent results now I have to use incognito or change the search settings to be "verbatim." I hoped things would get better, but years later things are the same.
DDG gives more predictable results. It reminds me of why I started using Google over other search engines decades ago.
That solely depends on your definition of "good".
These comments have plenty of examples where DDG is "better" in terms of usability compared to Google, which is also an important factor besides search accuracy.
Imho using the search history as context can also lead to undesirable results, so can funneling people to overly popular results which still can be wrong, had both happen more than enough with Google.
It's a trade-off between spending time arguing with Google if I really meant to search for that word, regardless of " usage, and having to look at more results in DDG until I find the one that I was looking for.
Personally, I value my privacy more than the small amount of time tweaking my search query to get the results I actually wanted vs. the out-of-context results I got. I think anyone using DuckDuckGo is of the same opinion. When you think about it: total loss of internet privacy for some slightly better search results? Pretty massive trade off of with some serious consequences.
However, I do get your point. Without that extra context, the search results can only ever only be so good.
It might be very good. But can such a system compete with one that does use search history as context?