That depends how they got wealthy.

Did they steal everything outright? Someone is worse off in that transaction. (Or everyone a little bit worse off if it’s government grift).

Did they create all that value themselves? Might be fine - positive sum games do exist.

Did they create some system where a bunch of money flows just to them based on the labour of others? Maybe it depends on the details, like how much the labour is paid.

I think Piketty’s point was around capital and wealth tending to accumulate unless something forces it to disperse. This can get worse over time. The last couple hundred years were relatively “good” due to the way revolutions and WWI and WWII basically eliminated many of the wealthy families in the west, a couple times, and the post-war societies were “reset” with good equality that has slowly eroded since (due to insufficient “friction” to prevent accumulating extreme wealth over time, such as high loophole-free wealth and inheritance taxes). Or so the theory goes.

Building on that, when you get extreme wealth you get individuals with power to affect policy for their personal good. Some will choose to be selfish (it’s human nature). Policy shifts in their favour. We end up going in the opposite direction to that since the Great Depression - which really was a collectivist culture of everyone getting a share of the wealth of the nation, rather than being screwed over by rich and powerful folks. (McCarthyism somewhat put the brakes on that in the US in particular, though, which is why you can get e.g. free health care elsewhere in the west).

> Did they steal everything outright?

In a market economy, theft is illegal.

The Great Depression was not caused by rich people. It was caused by the Fed and its failure to understand the relationship with gold and dollars, and mismanagement of the bank reserves.

WW2 was not caused by rich people, nor by inequality.