Just to over simplify the idea if lets say the market valued the average stock at a PE of 11 and everyone found stocks a wonderful investment. And so more and more people buy stocks and with everyone finding stocks wonderful they keep buying and after awhile the market is valuing the average stock at a PE of 14.
Within the market there is tons of variation those things of course are not nearly that simple, but the idea I think holds. Well if you look back at historical data the returns will include the adjustment of going from a PE of 11 to a PE of 14. Now maybe the new few decades would adjust from PE of 14 to PE of 17 but maybe not. At some point that fundamental re-adjustment will stop.