It's not main currency, but if you call it as a digital currency, as another method of payment yes it is. And I think it has nothing to do with the stable or unstable price because the technology that used by bitcoin is what matter regarding to your question. Perhaps this https://www.cnbc.com/video/2017/11/06/japan-made-bitcoin-a-legal-currency-and-now-its-more-popular-than-ever.html can help you out where a big country like Japan use the technology and how it woks rather than speculating about main currency of economic world.