Bitcoin is already the first most talked crypto currency, just few years after creating it. So we can predict it will be even the most used currency worldwide after 20 years from now.
That's a bold prediction, I think you mean majority of the world's population are going to use bitcoin, that would be possible but the success rate is not that high. We have seen how the government react about bitcoin, they just rejected the ETF application which everyone thinks could help the progress of bitcoin. Let's think that without the support of the government, massive adoption will not happen and despite of that I still believe bitcoin can still live to serve a smaller portion of population in this world like what is in the current situation.
makes sense I doubt it will ever be the most used especially once it reaches 21 million made and no new ones are created. It has been going up so much lately though you may be right (other than the recent drop). I imagine that just like how when stock prices are super high right before a split and everyone is trading I bet it will be popular to the end and popular after but unlike stocks bitcoin don't split so they usage might fall off eventually.