NOW for the way too long answer!
Historically, over the long-run, stock prices tie into corporate profits, which is a percent of GDP, which is tied to population, inflation and productivity. historically, we had averaged about 6% rate of return over any 40 year period. That equated to 2% population growth, 2% inflation and 2% productivity.
Some will promise 10% ROI, but that has never been close to happening over a period that matches a worker's working years 40-50 years.
What we really care about is inflation adjusted purchasing power of our investments.
If we go back 40 years to 1980, we're talking an S&P of 110. Adjust for inflation puts that at 365 in today's dollars. Using money chimp interest calculator inflation adjusted-ROI of the S&P over the last 40 years is 5%. Much of that is multiple expansion (stock prices disconnecting from corporate earnings).
If the medial household income is $60K and they put away 10% of their income for 40 years, at 5% ROI... back to money chimp compound interest calculator... $761K. Not millionaire, but close...
HOWEVER, excluding out recent period of multiple expansion, historic inflation adjusted return from stock is closer to 4%, which is enough to drop it to $600K.
HOWEVER, we don't earn money equally over our lifetimes. When we're in our 20s, median income is closer to 40K, not hitting the median until well into our 30s, reducing years for our "average" savings from 40 years to closer to 35. That change drops our inflation adjusted total to $450K..
Now, here is where it really takes understanding macroeconomics....
EVEN IF everyone put 10% of their household income into stocks, for 40 years, and got 4% inflation adjusted ROI, not everyone could have $450K worth of stocks.
$450K * 150 million households = $67T Total of all USA publicly traded stocks is $25T.
Okay, but is bullshit number since not every household will have that $450K at the same time. True, you build up to that over your working career, then burn it back down over your retirement... meaning the average would be half that... okay... $200K * 150M = $30T. Closer to the $25T total stocks.
BUT!!!!
80% of stocks are owned by the richest 10%. So, that leaves $5T of stock to buy with the $30T total we would have to have in the market. Work backwards. Divide the $5T in stock not held by the top 10% by 150 million households = $33K each average. Double that by the time we're ready to retire. Most households would be lucky to hold $100K of stock by retirement time.
So... no. We can't all invest 10% of our income, consistently over our working lifetimes, and enter retirement as millionaires.
This leads me to my saying "I don't want an economy where someone can succeed. I want an economy where everyone can succeed."