The issue for me is I don't know of other good places for long-form discussion and good link aggregation in general. Most of the time in smaller subreddits there aren't really any issues.
What alternative is there to /r/AskHistorians? Etc.
I mean, their tagline "the front page of the Internet" screams arrogance to me, in the face of what Tim Berners-Lee envisioned the Internet to be, and yet it's probably not wrong at this point.
The absolute rank doesn't really tell you where the next closest competitor is.
Digg was pretty big for the time. The competitor I used the most back then was slashdot and I'd say digg was considerably bigger.
I have an idea that the future will not be in content silos and volunteer moderation by power hungry people... but I certainly don’t know how to get there so I’ll wait. I suspect the future will be “subscription lists” you join that block things people you generally agree with flagged, but this may bubble issue like we have now.
There is no popular alternative now, but something could appear eventually.