Just a few thoughts: unkillable companions is pretty immersion breaking. So. I thought of some fun ways to get around that, other than competent AI.
Dogs.
Well, Other than just finding a new one. We could perhaps just buy/breed dogs. Maybe you could make a business out of it? Those settlements you can make? Add a training pit where you can breed/Train dogs. Maybe have a guy do that stuff for you.
Robots
Pick up all the pieces. Put them back together again (superglue and duct-tape permitting). When your robot inevitably explodes, you can pick the pieces back up (or at least the memory core/brain) and then rebuild it yourself, repair it yourself, or upload the brain into a new robot.
Hired gun dead? Hire more. Hired mutant dead? Get a hired gun and give him a bath. Slave dead? Enslave more. (randomize the names, voices and skills of these guys.
Your child that's heavily implied to be a major story character. If your son/daughter dies, just get busy and make another one. Simples! Don't have a wife/husband anymore? Make a new one (not by some [censored], shallow, poorly implemented marraige system; but rather you just make a kid, hope it's the right gender, and raise it to be your future spouse. The true wasteland way)