OldFatGuy: Wow, part of the fun of Fallout NV was seeing how long a companion would live wandering the wastes with me.
Bassackward decision. You have two groups, one group re-load if a companion dies, another that wouldn't. If you allow them to die, both groups can be made happy. By making them immortal, only the former group is happy.
Why does this decision not surprise me....
gunsynd: "Like everything else, it's entirely optional. So, you can have a companion with you and it can be the dog, it could be other characters we haven't shown yet," he said. "Or, you could have no companion."
OK, my bad, you have THREE GROUPS. One group would like to reload when a companion dies, another groups would like to allow their companions to die, and another group would avoid companions altogether.
If they let them die, then all three groups would be happy. By making them immortal, they ensure one group is not.
ADDED: BTW Bethesda, here's an idea. Why not have immortal companions be a configurable option in Game Settings. Those that want to experience the "real world" and all of it's consequences would leave it unchecked, and those that don't want to go to the trouble of reloading when a companion dies would check it. Everyone happy.