It all depends on what a person means by "Mass Exodus".
If you mean within a period of just a couple of months that millions of JWs will walk away from the religion - than I'm with Simon on this and I don't think it's likely to happen.
But if by mass exodus you mean the churn becomes so high they start having negative growth - then I think they're already there. I believe they have been fudging the numbers for the past couple of years by moving the goal post of what they call a "publisher" and they won't be able to hide their reducing numbers for much longer.
I also think it's important that we make predictions (not proclamations). Because over time it allows us to look at what we first thought vs. what's actually going on and produce more accurate models. My own prediction for a "mass exodus" is that in fifteen years there will be half the number of publishers. And I think such a prediction is worthy of the title "mass exodus". But that's just me. It seems like very few others on this forum are willing to make specific predictions about where the WT will be in the next ten to twenty years.