Why do you believe there is such an assumption? Elon Musk assumes people will try to achieve a post-scarcity society and he’s nonetheless gone from living in a YMCA to being the richest man on Earth.
Again, the assumptions don't tell us anything meaningful. If, hypothetically, we did achieve post-scarcity and pushed out capitalism in doing so, so be it. If that were to happen and then "bad actors" took back control of the capital, returning us to capitalism, so be. These models don't have to be ever-present, they don't have feelings, they are only descriptions about the state of the world at a given moment in time. Whether or not these forces described are in play is irrelevant.
He comes from money, and would have had no problem living anywhere he liked.
Just read his wikipedia page:
> In 1995, Musk, his brother Kimbal, and Greg Kouri founded web software company Zip2 with funds borrowed from Musk's father.
If the YMCA story is supposed to signify "rags", then it's a carefully constructed image and nothing else.