Reddit was not a clear alternative to Digg, but it only took a short while for the transition to occur. it's not the consuming user base that needs to transition, but the moderators who are willing to manage subreddits and users who spend the time find and share content. If they move somewhere else because Reddit becomes too difficult to use, the rest will eventually follow.

Reddit itself isn't a terribly complex site to implement as is shown with the relatively small staff they have for a very large user base. Until relatively recently you could download the source code from Github and spin up an instance yourself. The actual "work" is being done by the unpaid moderators and contributors.