What formula do you use, if we have 1 BTC today then it becomes 40 thousand in the next 7-8 years. I think it's irrational. It is impossible for 1 BTC to become 40 thousand BTC in the future. If the price increases in the future it might sound reasonable if we have 1 BTC today at a price of $40k and a few years later the price rises to $100k we will make a huge profit. But I'm not sure if crypto will become a medium of exchange to replace fiat currency in the future.
Is he a mathematician or what? Because, he has his own formulae to calculate stuffs. A 40k usd/btc is just an example but reality is that btc can cross over that amount. Imagine how much btc will that be equivalent to but no I do not think you can now buy anything but for sure the prices of the items are also going to be adjusted and they can look a little more pricy than their usual price that we are seeing right now.
Bitcoin is already a medium of exchange and depending on the user, they can treat is as a fiat replacement if they wont use fiats anymore but I think in the future, the situation will be the same. Fiats and usd are still the measurement of everything including btc.