Selling Bitcoin due to the drop in price doesn't help every investor who's truly a long-term investor, i believe doing nothing, or buying more if you have spare capital it always the best thing to do due to how BTC has always been volatile. Besides, there's the moment when we’ve seen 70–80%+ drawdowns in the price of BTC multiple times in the past, and the price eventually recovered and went higher to create a new ATH price.
The people who sold in panic during drops are always the ones who end up losing money, while those who held or accumulated the dip make more profit.
Sometimes holding without a target for a stop loss is a foolish thing.
Stop loss is only applicable to Bitcoin traders, but setting firm boundaries/holding time span and sometime exits when the reason for holding is broken is for long term investors.
So selling during a decline can sometimes also be a good move to reduce losses.
Selling during a market price decline is never a good move to reduce losses for anyone (if emergency needs are not involved) if we're talking about BTC investment unless you're talking about a shitcoin.
But indeed there are many types of traders.
Now, I understand you. The discussion is about longterm investors.