The problem is that it is impossible to guess in advance what will be the price after a time. We have to deal with the inevitable losses.
Yes, losses always happen, this just needs to be reconciled. The only possibility to never be mistaken will only be for the owner of the time machine, but since it has not yet been invented, the losses can happen to anyone. Moreover, the crypto currency market is very unpredictable, so you do not have to worry about it.