If you see it from other angle, BTC can replace if every online business starts accepting BTC for transactions. They will still need to convert it to dollar.
Could you explain what makes you think so?
Bitcoin is already almost fully legalized in a few countries (see Switzerland or Japan) up to a point where you can pay for public services with bitcoins directly. And did that give these countries any advantage in regulating or manipulating Bitcoin transactions or "production" apart from what any other country out there already has in this respect (let's call that "administrative resource")? How exactly does Bitcoin legalization give power or control over Bitcoin to countries going that way?