you could buy bitcoin for 1mio, there is no proof, that the bitcoins belonging to you,
and you can exchange them anytime back to bank transfer or cash
Until the next day when you 1 million is now worth 840,000 and the the day after that it's worth 922,000 and then 740,000.... Bitcoin is not a practical way to store money. By all means speculate and invest, but treating it as a mean of savings is not realistic. Then there is the cut exchanges take, and the faff of their verification processes for large amounts transferred to FIAT. Then there is a mater of where to deposit it, source of funds, capital gains tax etc. Granted you can cash out small amounts very anonymously. But 1 million if you want to liquidate it is no simple mater. I'm curious why you think it is viable?