everyone stopping trading gold would only happen if everyone stopped using gold, so how is it still useful. Imagine, if you would, a machine that can make as much of something as you want, replicating it from nothing. Put gold through it. Is there anything about the gold that would prop it's value up above the pure value of the effort required to push the button? If gold is valued because of both scarcity and subjective value, what makes bitcoin any different? It has scarcity, and people subjectively value it.