The value is in knowing they always have to watch their toes because it can all be over in a heartbeat. There is a very fine line between domination and irrelevance.
Digg, MySpace and Friendster weren't "over in a heartbeat". It took decay and lack of care with the platform, plus strong competition with strong venture capital funding. Plus, they were never as dominant or as entrenched as Reddit is.
Reddit is not going away just because you dislike it.
As I remember it, what happened on Digg was that there was a kind of friendly rivalry between Reddit and Digg. So everybody on Digg was already aware of Reddit. Then when the redesign hit, a meme spread that "This redesign sucks, try reddit". And so people did. And what they found was a much better community (Digg was more like a news website with a comment section).
The cautionary tail for Reddit is that what keeps people locked into a social network is community. And if they replace community with cat pictures, the whole thing will fall apart.
Yep. Also, what Digg did wasn't just a redesign, but also a complete rewrite from scratch in the exact way Joel Spolsky told everyone not to do one decade before [1]. It was buggy and missing important community features, and it was incompatible with the old website because they had changed the whole database too, so there was no way of choosing between old and new running side by side like you can with Reddit.
