What definition of rational are you using? Being a rational actor typically means displaying consistent goal-oriented behavior. Being lobotomized seems pretty irrational. It reduces your power and makes you less able to achieve goals (if you can achieve them at all), including basic self-care.

>Except it turns out that once you take the emotional side out of the person, what's left is merely a hull that doesn't care about anything, because rationally, why would you.

That's not what rationality is. What terminal goals one should have, which in humans is informed by emotions, is not a concern of rationality. Rationality concerns how to achieve terminal goals.