I find it impossible to believe that at all. The new Reddit UI is essentially just a reskin. The Digg "redesign" was an entirely new concept that radically changed the purpose of the site. The two aren't comparable at all.
Come on, are we really going to get this hyperbolic? The amount of people complaining about the interface is a vocal minority.
A very significant percentage of Reddit browses the old interface (I think 20% or 30%). These people are likely to leave and help create another viable competitor.