That's what the Mayans thought - what harm can a couple of people on a boat do?

> how is an AI going to affect anything in the real world?

At least two ways:

* Actuators, such as robots, industrial control systems, etc.

* By influencing humans: bribery (yay for crypto), blackmail, election interference, interfering with sensors (e.g. making it appear as if a nuclear attack was under way), and many other ways.

Either way, creating a biological agent that eliminates most humans (or food supply) seems quite feasible.