I think it's a question of "important" users. If enough content mods and creators leave, will there be enough to fill the role immediately?
If not - i imagine it will be pretty visible to people on the fence, which will only snowball more. Just as with Digg, this can happen quite fast, and i think a snowball is the only true way this would happen.
With that said, i think the most important part of the equation will be another "player". If voat.co (or others) are not able to handle the immediate traffic, then i think it's much less likely to succeed. Reddit had a lot of troubles handling Diggs load, but they managed overall. Voat has been taken down multiple times due to Reddit's against-user actions - so it doesn't seem likely that Voat will be able to handle the traffic when Reddit users need/want it most.
Personally i'm hoping someone can find a way to distribute this whole social content idea - but in a UX friendly, web based way. I don't think Voat, or anyone else, will be able to handle taking in the Reddit userbase without falling into the same pitfalls as Digg and Reddit. Something decentralized seems like the only solution.