Yes, this is a foolish decision. Although many people think that the price of bitcoin is very high now, I think bitcoin will definitely increase in the future.
I think it depends on the situation of why they sell their Bitcoin in today's price that is decreasiong. It's not foolish if they sell it for important matters or they really need that money. It maybe foolish if you sell it because you've heard that price decrease or a fake news that Bitcoin might not increase its price that result to panic selling. But we know Bitcoin price will increase so we just have to hold and wait.