He'll be either right or wrong
The interesting thing is that I hear and read so many similar opinions to the one expressed, so much so that I am not sure what's the real sentiment, euphoria or fear. As Ritholtz himself has pointed out several times, that fear in and of itself could be driving markets higher.
At the end of the day, I don't think it will be a change in sentiment, per se, that will turn this market into a bear, but rather something exogenous. What that might be, and when it might come, is anyone's guess.