I like the 1000x threshold because that is approximately the maximum possible fortune one can amass in one's lifetime off of ordinary salary work and extreme frugality.

1000x the median income would be about $80 million. Consider the highest-earning non-executive salaried employees - people who spend years in school in very challenging fields. People like neurosurgeons. Imagine if there was a couple composed of two neurosurgeons, and they earn very good salaries. They're also so frugal that they spend basically nothing. You have a pair of neurosurgeons literally sleeping on the sidewalk out front of the hospital. They live like that, and they invest and save every penny they can. The highest salaried incomes combined with pathological frugality.

Even if they did all of that. Even if two highly educated workers lived off nothing and saved everything, even then those people would still struggle to earn, over their whole life, a fortune that exceeded 1000x the median household income.

Such a system allows for a capitalism that actually does live up to the marketing. You're allowed to earn a fortune as large as your own labor and skills will allow. However, the only way to obtain a fortune larger than this is to get into the business of labor arbitrage - hiring other people and harnessing the surplus of their labor. I want people to be able to earn as much money from the sweat of their own brow as they can. But I don't want people to be able to hoard strategically dangerous fortunes by exploiting the labor of others. And 1000x the median household income is a nice even number that's easy to explain to people and that achieves this goal.