Earlier quoted context omitted.
Digg users had an attractive alternative to run to. Reddit users don't.
Reddit is really not that special under the hood. Given the opportunity it can be replaced. It just suffers from people moving issue most social networks suffer. A decentralized alternative Lemmy[1] is already quite feature complete and in many ways even better than reddit itself. It's just a bit of a ghost town and many of the instances are aimed towards and/or controlled by political/social rejects.
"just"? Reddit is the community, not code. Users aren't going to run off to Lemmy or any other site. It just isn't going to happen.