if it becomes a global single currency it will be very dangerous for the whole world because just a few would have the control of the entire market and the entire single currency by being a monopol
You are right, but there won't be a monopoly if the global single currency would be something that no entity can control(decentralized). This is where blockchain comes in.
Bitcoin as a single currency is an awful idea. It would collapse economies all around the world, there would be nothing to measure it against, and not to mention not everybody would have access. That would fragment the world wide markets and for that reason it can't happen.
At least it can reach 6-7 figures and it has the potential to be the gold in digital money(since it is the pioneer) but for now I don't see it becoming an alternative way to pay stores globally in the future(because people use it merely for speculative purposes, and the number of stores that will accept it as a payment would be limited to stores that have access to it just like credit card).