My best argument to this that I would gladly be proven wrong so I don’t look silly being wrong. Is there any example in the past ~7 years, even ~10 years of any major cultural user content sites that had precedence like Digg, losing influence the way Digg and other contemporaries did?

The only examples that I can think of is Tumblr.

If there’s so many examples before 10 years ago. And maybe 1 example in recent times. Your point can still stand. However it also doesn’t mean much when the resilience of web giants are totally different between the Digg Web time and after that

Yeah I think also that we underestimate these massive sites by thinking they have not learned from Digg etc.