I don't think the Fediverse will suffer the same demise as Digg and Reddit, precisely because it's not owned by a profit-driven VC-backed corporation, but there are a couple of other serious threats to its longevity:

• Moderation. If the Fediverse isn't adequately moderated, it will quickly be overrun by Nazis, pedos, and spam. That's what killed Voat and Usenet.
• Funding. This isn't like IRC, where a modern server can support tens of thousands of users in its sleep. Running a system along the lines of Reddit or Twitter requires a lot of computing power, and that's expensive. Where's the money going to come from?