There is time value of money but if the coin is a well supported coin with good concept and utility there is no need to sell @$90 or $50. The best thing to do in this knd of the situation is to leave the investment and start a new one at the $50 price if have enough capital but if capital is an issue is better to sell @$90.
True - if you add time value of money, return will be way worse. However, most of the coins are without any utility or use - simply created for scamming people or for attracting traders. That's why it's gonna be hard to watch how the balance declines when the asset decline from $90 to $50, and you can't even sell that.
Good point cause 80% of all the newly created are hype base and this is the expanse where every crypto traders need to have knowledge not only to read the chart/candle but also have the awareness of the perfect investment in the which is not hype base.
I don't trade often cause i don't like spending much time on trading but i surprised some traders usually complain they only make losses whereas in the knowledge they lack.