Yes I've read about that elsewhere, but they do still use a lot of other signals AFAIK. The more complex their algorithm becomes, the less inclined they will be to change it, and the beauty of only using user input is that you get an ever changing signal and people start to be less obsessive about their exact ranking, because it changes constantly. Of course it does introduce problems of its own with trust.

Re competitors, I think that's more about eyeballs than quality - Google is such a behemoth now and is entrenched with revenue from advertising - they can use their other properties and interests like mobile to feed search revenue. I really don't think they're at the top solely because they return the best results any more, because they've been without significant competition for so long.

I was actually more interested in the idea of injecting some random results (say from the top 10 pages), as that might significantly increase the relevance of their results for many queries, give them even better data from user input, and would make it very hard to game the system.

The idea that you could only use user input is not realistic. 15% of Google's incoming queries have never been seen before by the search engine. There's not enough user input available.