I think it depends on the price of the bitcoins and the inflation, you can not predict the money value of current day after 20 years. I will say that don't store bitcoins for 20 years, you never know where the bitcoin-economy will be after 20 years. For the best case, get 10 bitcoins, sell it whenever you think the bitcoin exchange price is at its best and then keep that money in fixed deposit for 20 years, this is more safe.