> no single person does or ever did own such an amount in liquid assets which he could sell transfer the proceeds to someone in a short period of time
You seem to be confusing three similar, but fundamentally different things: physical cash, liquid assets, and being able to transfer in short period of time.
Just because billionaire's assets are illiquid, it doesn't mean they can't transfer $10B in a short period of time. It simply means that portion of value will be lost during that transfer.
> What private person could have that much cash stored somewhere? And most importantly why?
Elon had $20B in cash (i.e. liquid assets) after selling 10% of his Tesla shares. He lost some value in the process, but afterwards he had $20B of liquid assets, which debunks your statement.
Arguably, same will happen to $10B of BTC, given that such transaction(s) would be public and will raise concerns.
And that's just Elon Musk, we're not even talking about western adversaries, like Putin or Xi, who have access to even more assets.
And if we're thinking of BTC simply replacing physical cash, then you can even build fractional reserve system on top of it.
Also, we seem to be discussing a magical world where BTC became the major currency overnight, with no time for financial system to adapt. If we go back to the real world, I could totally entertain a thought experiment of BTC replacing USD. Not that I support it, but again, few lucky billionaires is the least of my concerns, given that we already have those in the current system.