They have also, in my opinion, implemented mainstream and niche news so well (via subreddits) that they're not in immediate danger of being replaced by another aggregate news and/or niche site.
so was Digg, and Fark before it, and Slashdot before that.
Am I wrong? History shows that these sites are basically popular for a certain amount of time, and then they lose huge market share when they inevitably do something to sour the community-at-large (like trying to monetize).
Reddit figured out how to support subcommunities in a way that those other sites never did. In my opinion that's what's different.
See also: social networks. Every social network was a passing fad...until Facebook wasn't.
You meant it hasn't so far. That's not a dig at Facebook, or a prediction of death, but on the short timescale we're looking at (a few years really) it's much to soon to know whether they've stopped the cycle, or made it slower. And therefore you can extent that to Reddit, the fact that they've gone longer without decline doesn't mean they won't start their decline tomorrow, or the next day.
The context is a hypothesis that this type of site will grow and eventually die down - and assuming that hypothesis, it's too early to say whether Reddit has changed that trend or just changed how long it takes. I'd say that in another 10 years, if it starts to die then, it would have gone long enough to say it changed the trend. But right now, too soon.
Am I making (rough) sense?
I'd say they're not in immediate danger because they're not making a profit and most businesses that have the sort of money needed to support such an effort (ie the large traffic) don't want to waste money on something that's not going to make a profit. Indeed Conde Nast appear to have only recently and quite reluctantly allowed reddit some more resources.
Explosive growth in traffic, yes, what's the balance sheet look like.