I think it's more than just "one day I'll be rich"; it's part of the mythos of where wealth comes from. If your starting point is the American Dream, that wealth comes purely from a combination of ingenuity, luck, and hard work, either from the individual or one or more previous generations, then it's easy to see being asked to pay away a share of it as unfair. But if you see wealth as something that arises at least in part from the opportunities afforded by the structures and services within a society, then it's reasonable to expect that the society is owed back a portion of it, in part to maintain those services for the benefit of others coming later.

And fairness aside, there's the purely pragmatic concern that multi-generational wealth concentration is bad for almost everything— bad for democracy, bad for fairness in media, bad for civil unrest, etc etc. Is taxing wealth an effective way to manage this? Maybe, maybe not. But anyone who wants to critique it should probably be willing to support _some_ plan for controlling wealth inequality over the long term.

Doesn't the truth of this hang on what the definition of "success" is? Is ability to produce the occasional Mark Z or Jeff B "success"? Or is success just an upward slope on broad economic indicators like GDP?

I think the picture looks quite different if you look at things like quality of life indicators— and then it's (surprise!) nations like Denmark and Switzerland which top the list. I suspect it would be even more stark if you were able to measure class mobility— the number of people who have at least +x% inflation adjusted net worth over their parents at the same age.

So yeah, it's a little bizarre (and a very US-centric view) to just assume that "success" on a national level and socialist policies are mutually exclusive.