"The S&P has long been considered a reliable, long-term market investment that is expected to grow at a particular rate, i.e. Seigel's Constant."

I realize that you're directing your questions to mungofitch, but if I may, I'd like to throw in my two cents on the Siegel constant. I think that it has very limited utility. Siegel derived his constant from 100+ year returns, ending in 1992. In September of 2022 Siegel defended the constant by saying that he updated the data to 2022, and that the constant remained unchanged at 6.7%. 130+ year holding periods, while interesting, have little use for individual investors. In his 1994 book "Stocks for the Long Run" Siegel made the valid point that the period-to-period scatter in returns decreased as one moved from 1-year holding periods to 10-year holding periods to 20-year to 30-year. However by averaging all 10-year or 20-year holding periods he completely missed the huge role of starting valuation. The variation of 10-year or 20-year market returns with starting valuation is extremely important to individual investors. If one is looking at 10-year or 20-year returns the "Siegel constant" is not constant. Real, 10-year returns, for example, range from 18% to -5%.