I think its important to remember that digg was changing the way content was generated and promoted to users. They kneecapped the entire democratic principles of the site, and reddit, a site entirely based on democratic promotion, was waiting in the wings.

Reddit on the other hand is changing how content is accessed on their site, but not changing the visibility or generation of that content. And if 3rd party apps are very important to those users, it's hard to find an alternative to plug in to.