This is not necessarily true. Usually, when the market falls, you should leave quickly and you can not hesitate. Unless you are a long-term investor, you think the future of cryptocurrency is good, otherwise the first sale is the best way.
There are lots of up and down in the history of Bitcoin. One should never invest in a volatile market what he can't afford to loose. However the blockchain technology is here to stay. It's is the future. So only investing in a project that has a definite future makes sense..