Yeah? How many robots? What kind of robots? What would the AI need to survive? Are the robots able to produce more robots? How are the robots powered? Where will they get energy from?

Sure it's easy to just throw that out there in one sentence, but once you actually dig into it, it turns out to be a lot more complicated than you thought at first. It's not just a matter of "AI" + "Robots" = "self-sustaining". The details matter.