Yes, it's wrong, and it's wrong on such a fundamental level I wonder if people who don't understand it have even given it a minute's thought.
You've lost money regardless of whether it is realized. You can even find a very simple contradiction in what realization even is: if you bought a stock at 100, double up at 90, and now it's 80 and you sell some, how much have you realized?
Your total net worth is the same regardless of whether you thought you realized a unit of -20 or -10.
> If you stay put, it will very likely be recovered AND have grown in time.
You should think about what risk of ruin is here. If your investments keep going down, what's your move? Double up, because it will likely bounce back? I may not agree that the market is efficient, in fact I make a living out of the inefficiencies, but the degree of inefficiency is close to a rounding error: the current price is a decent estimate of the value, incorporating all known information.