I think the main problem here lies in assumption of the distribution over the various locations. $50k+ in Appalachia is good money and you might be able to save $30k out of $80k. One would not be saving and investing in many of the large cities and their suburbs (where more people live).
One thing to note is that the $1.5M-3M is not inflation adjusted and would be worth much less than it is today. It will result in better outcomes, but it won't make people rich, like many claim. $1.5M is just enough to keep a couple out of poverty who will be retiring at 65 years old, 35 years from now. This is especially true if people live longer and the cost of healthcare continues to increase much faster than inflation.
"(which is lower than what the S&P has historically returned by a fair amount)"
The next decade is supposed to be much lower. (Past performance is not an indicator of future returns).