Earlier quoted context omitted.
Buffett had a good quote about it once that I can't find now, but essentially during most of your life you will be a net buyer of stocks. Only at the end, during your retirement, will you be a net seller and only then will you want high prices. Until then, the less you pay for your stock purchases, the better your long term gains will be.
Another way of thinking about it is: on average, stock value over the long term (20+ years) is very likely to be in range of, say, 5%/year, plus or minus (actual number is not that important for our purpose here), after adjusting for inflation. If stocks have been on a recent runup, gaining, say, 50% or 100% over a period of a few years, then they are very likely to grow more slowly than average (i.e., revert to the…
This is incorrect. Prior performance of the market over the span of years has little to no predictive power on future performance of the market.
Your statement is like saying: Because I flipped a coin and got heads 10 times in a row, I'm more likely to get tails in the future.
While it's true you should expect the market to revert to the mean over the coming years, there's no evidence that it will grow 'more slowly than average' in the coming years to 'make up' for the hyper growth in past years. If you were a betting man (and a non-sophisticated investor), you should bet that the future years will grow at exactly the historical mean.
Edit: I did some analysis on historical S&P 500 pricing to validate my intuition.
On average, the monthly growth rate of the S&P in a month following a bear month is -0.39%
On average, the monthly growth rate of the S&P in a month following a bull month is 1.08%
You might argue that it takes longer than 1 month for the market correction to occur, so I've included the script and data set I used here for you to play around with: https://pastebin.com/F78pLUka. You can use any cadence, and will find the same relationship.
Empirically, you cannot time the market, which implies future growth rate is not affected by past growth rate.