"The mid-20th century was an era of mass prosperity in the US and parts of Europe, but it was an anomalous few decades, not the norm."

But to those living and remembering that era - it was the norm that they (we) compare with, so it is the reference that matters.

In the past you could find rich people on the battlefield. The last time America tried that was in Vietnam.

That is what has changed.

> How many times in history has wealth been less concentrated?

Mostly all of them! There have been periods where inequality dropped, but mostly it's been rising since at least the 1300s. I'm on mobile and can't link research, but there are a few papers that investigate this.

> As far as I'm aware, for almost all of history post-agriculture, wealth was highly concentrated while the average person lived in abject poverty (think: kings vs peasants).

And yet it was less unequal than now, an era where we've managed to use technology to concentrate wealth at an unprecedented scale. No longer is the richest person you know the king who collects your taxes next door, now it's a SV trillionaire on the other side of the world.