Digg still exists even if it's a shell of its former self. Tumbler imploded and moved to Twitter, which is trying its damnedest to destroy itself too. They don't disappear, they just slowly become irrelevant.

Reddit will still exist, but the content will continue to dry out. Sanitized for advertising and low effort for consumption. The thousands of small self contained communities that really drove the site will slowly die off. Subs that relied on outside moderating tools to ensure quality content will give up and move on.

As small hobby subs find new places to take residence I would absolutely believe reddit will become less relevant.