Well first he isn't really making a prediction. He is saying that if valuations return to their historic norms we would have low to zero returns. Shiller is also projecting low to zero returns if that happens.
Shiller & Bogle's math are almost identical. Both say that 0% returns are possible if valuations returned to their historic averages. CAPE and PE are both high. If they return to historic norms and if growth continues at this fairly slow pace then we would have no returns over the next 10 years.
Note that is a HUGE 'if.' CAPE and PE have both stayed above their historic averages for most of the past 2 1/2 decades. It coincides almost perfectly with the time frame in which 401ks and discount brokers have become more popular. More people are investing than at any point in history. So you could build a pretty good argument that PE and CAPE ratios 10 years from now will likely still be above the historic averages. I try not to make predictions. I'm just pointing out you could argue it either way.
Now Vanguard and other investment firms have warned that with valuations being on the high end(but not bubble territory) returns over the next 10 years might be closer to the 5-8% range rather than the 10% historic average.
http://vanguard.com/pdf/ISGVEMO.pdf
