Google isn't in a position to measure enough user metrics with enough accuracy to rely on them alone. Hence the use of links and so on.
Perhaps not relying on user metrics alone, but I think it has a big influence. I have cases of pages with no links, which have good onpage technical SEO, good content and are pages that visitors like, climbing steadily up in SERPs. The result is not instantaneous (the climb is slower than via the link building), but watching a number of pages like that over a year or so, I have no doubt that user engagement is an important factor.
I also think that if you do build links, these must come hand in hand with user metrics. In other words, building links may bring your page up the SERPs sooner, but if user metrics do not "validate" page's place, the ranking can deteriorate with the time.
I personally think that with over 20% of usage via Chrome, complemented with usage via many other Google properties (maps, other google APIs etc), Google has more than enough information on user metrics.