$BTC $ETH $XRP

MicroStrategy is crazily buying Bitcoin at high prices with the money of stock investors. Now he has nearly 500,000 Bitcoins in hand. I want to ask, who is selling these Bitcoins that he bought with the investors' money? Are retail investors selling them? Do retail investors have so many Bitcoins? Financing with one hand and buying Bitcoins with the other, how long can this model last? Can it continue indefinitely? 100,000, 200,000, 1,000,000, can it just keep going like this??