I wouldn't term it as a good or bad advice because of a simple reason - destiny! I strongly believe that destiny drives us all and if destiny says that I will die as a billionaire, I will become one! We as a commoner, don't know how and when a coin will reach its ATH and we wouldn't know when to sell it off to book profit!
I don't think it's purely destiny. Because you create your own destiny. If you work harder, you can die as a billionaire. It's not like you'll become a billionaire without efforts. Yes we don't knows what will happen to a coin for the coming days, months, and years. But if we think carefully and wise enough with our actions in selling coins, we can earn bigger profit than might even lead us to be a billionaire.
If you know how it works, we just to be smart about our decisions with our coins for a better result