That's unlikely though not completely impossible

But you should first ask yourself whether Bitcoin is actually going to become the only currency used by the whole world. That's highly unlikely on its own. Even if we hypothetically assume that ("just for fun"), it would be more about cracking Bitcoin cryptography rather than stealing bitcoins like it happened at Bitfinex. This is certainly possible but the question itself is like what would happen if someone found a "kill switch" of sorts to power off the whole Earth. Obviously, people would quickly switch to direct barter (start using candles)