We dare not make realistic predictions about the price of Bitcoin in the next 20 years. The price of Bitcoin depends on too many factors. Bitcoin recently hit an all-time high, and the Bitcoin price is currently at $41k. Usually, it is a better idea to buy cryptocurrencies when they are in a downturn. However, we all remember how it was too late to buy BTC when it was $20, $100, $1000, $20,000, etc. If the estimates come true, the price may run into millions of dollars. Bitcoin's potential is definitely strong.
What OP did isn't wrong, it's an estimate plus it's just a prediction and it's alright for a prediction to go wrong. Of course, it's good buy crypto when it's on a downturn but if you think about it, any time can be the right time especially if you're aiming for the long-term.
While what the OP did is interesting this is something that cannot be maintained, so some sort of adjustment is needed to give a more realistic projection.
However at the end of the day it does not matter, what matters is that on average the price of bitcoin is going up each year and as such the best moment to enter the market was yesterday, but since that is not possible then the best next moment is to enter the market now, after all as things are going you will still need to buy bitcoin in the future anyway so why not do it now when the price is cheap?