OK, so there is a magical device that provides you with deep contentment no matter what. But then, when you die and they cut your head open, the physical, biological brain has shrunk. And that's bad. Is that right? Is this what it's about?
Is the story actually arguing something? I have just been literally asking what it even presumes itself to be about. Is it the gloss above?
If it's about agency, I don't get it. It seems silly/facile to assert the "right" decision is the thing that matters without making the thing so magic it doesn't even make sense. Does agency stop you from getting hit by a runaway bus? Help you from being persecuted for arbitrary reasons? The story tells us they all, unfailingly, live "abnormally successful" lives. That's all supposed to be because of their (non) agency because of the earring device?
Is it actually contentment or are you fading away so much you cease to exist?
The story is not about physical brain changes, no. It's about losing yourself. It's arguing that to exist is to make decisions. Following the device too much leads to a life that looks successful from the outside but inside no longer has a person enjoying the success, just a puppet.