I wouldn't go as far as to say 40% is achievable every year. In the cryptocurrency market it seems to be. But it can't last for ever. Because if you apply that 40% to 100 billion and 50 years... Yeah, I won't even start calculating how many..I don't even know what that count is much more than thousands of trillions.

However, I like to take a more cautions stance on this. 8% is achievable for anyone. That won't make you a millionaire with 1 dollar, but it will make you a millionaire with 1000 dollars a month in a few decades. You can even make 8-12%, but when you go above those sums it get rather hard. You might be able to pull of 40% for a few years if you're lucky, but eventually averages start hitting you.

I've discussed compound interest multiple times in my articles and I like using the 8% value, because that can be achieved by multiple index funds in many countries.