I was on Reddit for 7 years, they had their share of drama, but none of it seems to compare to the current moment. I think it'll follow Digg's downfall, but probably last a bit longer. One of Diggs primary problems wasn't community driven. Their expenditure was reportedly 200k a month. An unsustainable amount of money.
Community exoduses seem to happen in waves. I think what happens is core users (the people who add the most signal to discussions) leave. What slowly starts happening is the quality of discussion gradually dips. In fact, it's been dipping for quite a while in default subs. These big dips though are usually enough shake to get a few more people to leave.
Ultimately though the biggest saving grace for Reddit is probably SubReddits. While general areas have long been degraded (/r/politics hasn't had a reasonable thread in years) the more niche subs have managed to maintain their level of quality. I think this will make the site last a bit longer then the normal rotational period, but as these niche communities grow mirrors in refugee areas, people will slowly stop having a reason to continue visiting Reddit.
If the niche communities leave, the downfall will then become exponential until some kind of equilibrium is found. Then, the site will last as long as the coffers allow it.