Why do I not have a "right" to tell someone that he is making a mistake? I have a right to tell somebody whatever I want to.
I'm not preventing anyone from buying Bitcoins. I'm just telling him that it is a mistake.
According to your reasoning you would also have no right to say that our fiat currency will collapse and therefore it is a mistake to hold fiat currency, as long as you can not see the future.
Makes no sense...