Hindsight is 20/20. No one ever knows when to sell until it's too late. How could we have known in August last year that the coin would reach nearly 20k? At that point, everyone probably thought it could only get to 10k at the most. But here we are, a year later and the price is on its way back down to similar levels (but hopefully not all the way back to the 3k area).
I remember the shock when it broke through 3k. Everyone thought bitcoin was doomed then. I was shouting sell at 6k. Then it more than tripled. No one could have known.