Median household income is like $83k. If you were in a median US household and you saved half of all your income every month into something that had a stable 4% return, in 40 years you'd have ~$4M. Not even halfway there.

You'd need a little over 9% of stable returns over that 40 year timeframe to hit that $15M target.

The S&P 500 returns average 7% in real dollars (not inflated dollars).

My calculation makes that about $9m over 40 years.