Yes, for equity prices in particular he talks about P/E ratios (among some other metrics like corporate profit margins), and so you're right, it would be possible for this to mean revert by prices holding stagnant and earnings catching up. However, as far as I can remember (primarily because a big emphasis of his was how unchecked bubbles can cause a lot of damage on the downside) all the historical bubbles he studied (something like 50) always crashed with a big price drop. Not 100% sure though, which is why I was curious if you had any contrary examples.