Point is interesting. S&P 500 bell curve was same for nearly 100 years (till 1990). After that 2000 looks more like an aberration and slowly its getting back to the old bell curve. Interesting to note that except for few humps there is not much PE expansion on the long term also. If history is to repeat itself after 1929 next major PE event came only 7 decades later.
Another interesting thing to note is that if its S&P 500 or Nifty the PE stays in the 14-25 range for most of its life.
For my own personal view. I will consider PE 26 as exit levels for now.