Is it crucial if they DONT do it? What would happen ?
It is crucial
that they don't do it. Because if they do, they won't be able to spend their coins anywhere, or sell them, since nobody else is running their code. Their coins will be worthless and they'll have wasted their money mining them. It makes no difference whether they have 51% or not. Fortunately, miners are smarter than OP and won't do this.
is that true?so it happen just on people who mine bitcoins?or it also will happen to us who buy coins directly form miners?is this called "bitcoins classic" is still dont understand about that.