It’s been assumed because that has been hung overall recent trajectory, but societies decline throughout history.
This phenomenon stopped a long time ago in the USA - with each successive generation since the 1940s, wealth has concentrated further in older generations.
Of course that's largely because the supply of various asset classes (i.e. housing) ceased to grow at their previous rates, so prices have skyrocketed, and the capital gains accumulate to the older generations who own the most property.
People today are MUCH better off on just about every dimension than in the 1940s!
You'd rather go live in the 1940s? Or do you think someone living in the 1940s wouldn't rather live today?
He never said living standards were better in the 40s.
Sure, but that wasn't the premise. Young people (< age 55) today have a far smaller share of current total wealth than young people did of total wealth in 1940.
And until 1940, each generation accumulated a greater share of value than the generation before. Post-1940, each subsequent generation has a smaller and smaller slice of the pie.
Our society is based on the promise to our children that they will be better off than what we were.
That requires growth.