I've been thinking about this a little bit recently, most directly because of Twitter. I've seen this debate play out so many times over so many companies: "this is a huge mistake, [company] is done for", "nah, they'll just keep going on like nothing has changed", "this is another huge mistake...", "most customers will stay"... and then,
finally, one day, many years later, someone is the last customer at that company, long after the company has completely fallen out of the news cycle.
I think we like to talk about Digg because it is unique in this regard: the change was rapid and noticeable, due to an intersection of factors (including the existence of Reddit and the ongoing feud-in-jest between them that made many Digg users aware of Reddit).
But most often, I think what happens is that a company enters a long period of irreversible decline: they screw up, forget their purpose or core mission or whatever, but they don't immediately collapse. What happens instead is they become a revolving door for vulture capitalism, and it becomes utterly impossible for anyone to turn the company around. Depending on circumstances, they can coast along for many years, frustrating doomsayers and boosters alike. We could call this, perhaps, the Radioshack Inertial Effect.
So, sure, it's not like Reddit's entire userbase is going to evaporate in the next 12 months. But, Reddit's already been declining for a while, and this isn't making it better. The people at the top probably see the writing on the wall and are looking for their exit. It'll still be a brand in 5 years, 10 years, maybe even 15, but it's never again going to be better than it was yesterday.