Google has gotten very good at weeding out the hacks and tricks that black hat SEOs use (PBNs, spammy links, generated content, keyword stuffing etc.)

These days the most consistent way to get lots of traffic is to provide lots of value and help people.

Google can measure whether people bounce and keep clicking around for better answers to their queries.

I'd really like it to be the case but that requires an indexing algorithm where usefulness as a metric is defined as an objectively quantifiable property of a page. Is there such algorithm?

None of the search engines I use can do that on every kind of content out there. Google works well for software development related stuff and last minute news. Bing is good for video search. Yandex hardly ever censors pirated content which is useful in that niche. DDG works like a lottery where sometimes it is the best among all, but sometimes it shows garbage.