And the whole system will depend on the miners and if there gonna be any problem with them, then the system will suffer. And another thing is that when people will see how volatile bitcoin is, they will try to exchange it to more stable asset that will dump the price of BTC as well.
I guess we have already talked about this before. All the bitcoin will be mined by the year 2140. At least according to this source.
So at a point in the future, we can pretty much see somewhat stability in BTC price. And the thing about world currency. It is already considered one. Not a legal tender, maybe, but it will be widely accepted across the world. Hell, it is already accepted in many countries as a legal payment method.
Source : Google
So in very short time, we will be able to see more and more countries joining hands to legalize bitcoin in their system. So in order to make that use easy, people will come up with new technology and develop existing ones to a whole new level. By that we won't have to face many problems that you have mentioned.