The answer to this question seems obvious. There are no 42 year olds that retire by merely "saving and not over spending" when even one child is involved.
Your brother in law did not merely save and not over spend, he built a business and sold it for a large payout.
Now our spending is much higher but the growth of our investments over the last 10 years has been incredible and outpaced that by a large margin.
For a while I thought some startup stock I had was going to fund a lavish retirement but then AI largely destroyed the value of those shares so we're just planning on the boring index funds.
I said nothing of investments and growth.