I think you've entirely missed what I'm trying to say.

If your objective is to expend as little energy as possible, it would be irrational to expand into space - that's a waste of effort.

If your objective is to experience as much of Earth as possible, it would be irrational to expand beyond the solar system - there's nothing there that could aid experiencing Earth.

If your objective is to exterminate all life on Earth, it would be irrational to leave the planet - everything you need to accomplish your objective is right here.

"rationality" is when you understand the impact of your own choices. "irrationality" is when you act in a way counter to your own goals, due to misunderstanding or lack of self-control. Whether one is rational or irrational is entirely separate from what their goals are.

Creating a non-human intelligence necessarily means it might have non-human goals. It is absolutely not the case that those goals would require leaving the planet, or be optimally accomplished by leaving the planet.

In other words, being intelligent does not mean somehow being interested in the same things intelligent humans are often interested in. Your argument is critically flawed because it assumes that only certain goals are "rational goals", when in reality an intelligent being could have any objective, including objectives that are alien to most humans.