It all depends on the situation, maybe sometimes you need to sell on a fall in order to save at least part of your funds, this may apply to force majeure cases, like the fall of Luna, but this is really an exception to the rule. A trader who trades for a long time and receives a profit from this can be called an experienced trader, which means he follows the rules, and knows how to do it. If for a long time the trader has only losses, then this is just a gambling and it will not last long.
LUNA is a very specific example, because in this case, not only traders, but even venture investors suffered, because the price fell even below the ICO price. Investors do not constantly monitor the market, they had to act as quickly as possible, the collapse was very fast, so I don’t think that all investors put stops and managed to get out during the ongoing fall. No one just expected this, and it is also impossible to prepare for this, because such situations with large projects, especially with billion-dollar projects, occur once in a "century".