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Bitcoin can make a lot of people rich, you know that bitcoin's existence, sustainability and main nature is to increase over time. as part of a $40k investment process, bitcoin doesn't make you rich quick, it boosts your awareness of yourself. and education about financial management. from there you have more opportunities to get rich.
I don't refuse what you have said about bitcoin but here, op wanted to know only one currency name that would make him billionaire by grow his capital. I agreed on the point of bitcoin made people riches even Millionaires & will keep making in future but dont think it is possible for bitcoin to make 1 Billion dollar from 40k$ only. But that doesn't mean i suggest to avoid bitcoin. So my humble request to be logical
I think that if the intention is only to choose one investment, it is clear that the choice is in bitcoin and it cannot be avoided anymore because so far bitcoin has been tested and everyone has acknowledged it.
with the current bitcoin value at $ 30K, you will get 1.3 BTC, and if indeed there will be an increase in BTC up to a price above $ 100K [the next halving period], so that an investment of $ 40K has turned into $ 130K and clearly gets extraordinary profits ($90K).
so it can be said that investing in bitcoin is very promising and of course highly sought after, but it does take large funds to generate large values as well.