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.

It's a fallacy to believe that making the rich less rich makes the poor less poor. There are a lot of things in between which must go right.