It can absolutely happen. The population is primed. The same scenario doesn't have to play out for the end result to be the same.
Digg had some mildly bad policy before their exodus. Reddit policy is failing in a much more dramatic way.
If someone comes out with a viable alternative with, perhaps, one or two notable improvements there is enough froth to push people to a new home.
I personally hope it fragments. The slacktivists and amateur political wonks to one corner with the news, the popular entertainment to the other, and the learners/hobbyists/special interest associations to yet another.
Reddit is the Craigslist of 2015. It is doing too much and it is ready to be picked apart.
Sometimes I feel that human beings are tough to please. Once they get what they want, soon it loses its shine. Perhaps it's OK, may be that's what keeps us going. But does that impact our happiness levels? Not sure.