This is just the natural lifecycle of all things involving humans. Why people want the same stuff to remain dominant forever is baffling to me. I can imagine nothing worse. Reddit dying off to leave space for something new and fresh is a good thing.
Social media sites do seem to go through the cycle even faster though--probably because they're essentially digital nightclubs. We all know of that one hip club that everybody waited in line for in our 20s, and then suddenly the drinks get too expensive and the crowd gets older and uncool...and then suddenly there's no line out front anymore. And there's a new spot in town where the young people cluster.
Before Reddit there was Digg. After Reddit, I'm confident there will be something else.
It's more about the business side than the human side, the social need will always be there but the business aspect taints it. At least the modern corpo "let's IPO and become billionaires, fuck the people, and chase endless growth" taint it more so.