Make it a billion and it would still be strictly better for society. Hell, burn the money in a pile if you object to it being given to the state and it would still suffice. You don't need to completely remove inequality, or even get anywhere close, you just need to prevent the unilateral exercise of such tremendous power. Power must be decentralized.
The irony is that if you want to believe in the power of a market economy (which I do, or at least did before the modern era), you must oppose the massive concentration of wealth because it distorts our markets to the point of irrational uselessness.
There has to be some kind of limit. Take an extreme example. Would it be OK to have a single individual (or company) worth $1e24? A septillionaire could give $1 trillion to every person on the planet every year for 100 years and still have 99.99% of his money left. How much power and influence over the world would someone like that have, while the rest have comparatively close to zero? I think we all agree that wouldn't work for a functioning society. We probably also agree that $1million is probably too low a limit. So it's not a question of whether there should be a limit but where that threshold is.
I don't disagree that there has to be a limit. If the house of Windsor could own most of the world in the twentieth century and lose it, there's clearly a limit.
What I disagree with is the benefit of an explicit limit that's formally set once and can't be updated. And if it could be updated the limit would be constantly ahead of where the richest person is now and meaningless. Markets are an ecosystem, ecosystems are self-correcting. In large part, markets are the self-correcting behavior of humans valuing tangible things writ large. The market sets the limit, the idea of an autocratic limit set from outside such a system because we don't like the signal it's sending just strikes me as hubristic.
The minimum wage is a limit "set from outside" and most people (besides very strict idealist Libertarians) agree that it's a generally good idea. We might disagree on minor implementation nit-picks.
I think it's a mistake to single out one single economic system: specifically unlimited, unrestrained free-market capitalism, and frame that system as the universe's base, "natural" system. Doing so conveniently lets you frame deviations from that system as "unnatural" or "autocratic". Unlimited free-market capitalism is not some natural, universal frame of reference--it is a deliberate choice made and enforced by governments/people, and that choice can be un-made by people.
We mostly agree that a minimum wage that can change with political pressure is a good economic policy. Nothing but reasoning and political will prevents us from agreeing that a maximum wage, or maximum net worth, could be good economic policy.
If people are free to leave their current polity, then governments still need to compete for people wanting to live within their domains and are subject to a free policy market. In other words, the political pressure and good economic policy you're referring to are contained within a free market, not the other way round.
In fact, even if you're arguing for a state's right to ban emmigration, that's just an additional difficulty to voting with one's feet. People still left East Germany despite it being illegal, because West Germany was outcompeting it in a truly unconstrained market of where people wanted to live.
Million is not much, at very reasonable 2% or 3% it would be 30000 a year. Or just burning it straight up 50000 in 20 years. That is actually not that much money these days.
5 or 10 million starts to look lot more reasonable limit.
The irony is that if you want to believe in the power of a market economy (which I do, or at least did before the modern era), you must oppose the massive concentration of wealth because it distorts our markets to the point of irrational uselessness.