It's not going to do it anytime soon or likely even within 15 years, but it could in our lifetime I reckon. I think more economic instability in certain countries and maybe another financial collapse/crisis will happen soon rather than later and that could bring in many new people to bitcoin which could have a snowball effect.
absolutely not. there is no point in thinking it ever will replace any major fiat currency. just stop thinking/hoping that it will happen. it will not.
Why will it not? If in the future bitcoin keeps going up and fiat keeps going down which is what happens when they print more of it on a whim people might wise up. Money should grow over time not devalue and bitcoin could solve this and that's what's great about a deflationary currency.
Speaking of currencies, they are established by our government to have their own kind of money. There are many countries but we have different currency with corresponding value. If bitcoin crypto currency replace the dollars it might not be happen since it will affects the other countries currency.