This is a poor comparison but I think it helps to outline the problem. Its one thing if the player dies for whatever reason, because in 99% of cases the player can learn and given that same situation not die. I would even go as far as to assert that most player deaths are, situational speaking, miles apart from companions. Basically what im saying is the player does not die from the same reasons as companions the majority of the times. This while a part of the problem is not the whole problem.
The whole problem is that AI's that work for you are well...strategically terrible. Combine that with the fact that most companions have poor combat stats, you end up with situation of looming death for your companion. This leads to countless times your companion just die's in hilariously bad incidents. Because enemies are not limited in their combat abilities, they are designed to use all resources given to them to kill you and you them. As such you are designed by the dev to do the majority of the damage to hostile enemies, and those working with you are designed to "help".
The whole immortality argument is based on the fundamental situation that the AI's are so bad, that its entirely possible for them to die 5 minutes after you get them. But lets be real here, if you actually like your companion, 9/10 times you will reload to set your self in a position to combat the situation before your companion dies. If you don't like your companion, then in the fight they die in you really could care less, and it turns out to be a situation.
Immortality just takes out the middle man, you don't have to reload (which imo is worse than your companion fainting, come on now your PC tanks rockets to the face and shoots up drugs to fix it, your telling me your companion fainting is "unrealistic" "immersion breaking"?) if you like your companion and if you don't like your companion then just don't use them. Rather simple really.