I agree that having an ever-growing pool of immortal aristocrats would be non-fun. On the other hand, having my newly recruited siege experts, surgeons, and other totally inexperienced newbie companions repeatedly getting killed on their first battle is also non-fun (and pretty unrealistic--people like that would have been kept away from the the front lines in any historical situation). It was especially annoying since it was impossible, in game terms until yesterday. I started my campaign with "hero death" checked off, and lost three companions (with all their gear) before figuring out what was up.
There are solutions to this, I can assign companions to an unused troop slot, along with some tough troops to keep them out of trouble and send them to the back. This will probably work, and might even be arguably more "realistic" in some way (although I am not sure how they will make much progress that way). It will also be time-consuming and not at all fun. Dysentery and extreme body odor might seem more "realistic" for the period too, but I am not looking forward to the day they can be included in a game, either.
In the long term, having some way of culling an infinitely growing pool of nobles might be needed. In the short-term, having no better and less distracting way of keeping vital support NPCs alive than fiddling around with micro-managing their non-participation in a battle seems less than optimal.