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.