A lot less than most people seem to think. By far the best predictor of someone's wealth is their parents' wealth. By very far. The vast majority of wealth is generational.
But even if you believe in upward mobility, the point is that needs to happen first before you possibly have time, energy, and money to devote to your passions. It's not backward; if you'd argue "no, succeeding in your passions is what enables upward mobility", then you (A) are thinking of a very small subset of highly marketable passions, and (B) have identified the catch-22 that makes upward mobility so uncommon.
Do you happen to have any studies to back this up? It makes sense that parental wealth would be one of the strongest predictors of wealth, but I know of at least one study that has demonstrated that 79% of millionaires did not receive any inheritance from their parents.
The prediction does not hinge on inheritance (most folks manage to start their own lives, careers and families well before their parents pass), but on the upbringing, milieu and economic as well as social capital available.
"You're likely to stay in the social class you were born into" - is basically what the predictor means.
But even if you believe in upward mobility, the point is that needs to happen first before you possibly have time, energy, and money to devote to your passions. It's not backward; if you'd argue "no, succeeding in your passions is what enables upward mobility", then you (A) are thinking of a very small subset of highly marketable passions, and (B) have identified the catch-22 that makes upward mobility so uncommon.