I don't think it's the viability itself. The many options also play a role: back at the digg exodus reddit was the only alternative to a large extend or at least the only alternative most people were aware of.
But a new platform technology paradigm entirely might be better (decentralized).
In order for this to be Reddit's "Digg moment", there needs to be a viable candidate to switch to.
There isn't, as far as I can tell.