Today bitcoin is already known worldwide. So we dont need 6 years to be able that all people will be using bitcoin. Were living with advance technology today, and were still uprading every year. It means that even next year bitcoin will be use by almost everyone.
Lol though users bitcoin bitcoin is already quite a lot and is quite famous but not necessarily all of humanity will be using bitcoin? of course bitcoin has no competitors or hater of course they will not use bitcoin?