Even though such a replacement is unlikely to happen anytime soon, it is a useful calculation because it puts a ceiling on the value of a bitcoin. It shows that the value of 1 BTC cannot reach $1 million (in today's dollars) as some people believe.
Bitcoin's current value is 17% of that theoretical value at 100% replacement (337k), but current adoption is nowhere near close to 17%. This is because Bitcoin's value does not depend on its adoption as a currency, so it can easily reach $1 million just by being "digital gold". Kinda like the real gold's marketcap is higher than base money, despite nobody using gold as a currency.