Well, i do not think that it is stupid to sell Bitcoin because it is one way to earn more profit. It is also a way of investing in other terms of Cryptocurrency and that means buying alternate coins. We can say that Bitcoin can be number 1 for another 2 years but we really do not know how long will Bitcoin can stay number 1. People are still developing it and that is a good thing for all of the users.
I think not as stupid what they think because most people are selling in order to divert their investment to other coins once they feel that the cryptocurrency invested is not playing.