So my question is can bitcoin be a main currency of world economy?
It will become the main currency for crypto economy but not for world economy.
There's a long debate on how will bitcoin going to overtake the currency worldwide but there's a long way to go before it will go there. Dollar will stay and bitcoin will be it's rival and alternative choice for people who wants to stay anonymous.
Yes i do think its hard for bitcoin to be world currency, but in crypto world bitcoin will be more popular and become the first coin to be accepted for new ICO and also ICO would be really the best option for startup company to fund money. Bitcoin will become more likely gold and become good asset for investment.