"Yeah but it's still immortality. They get knocked out, then the player just hides until they wake back up. The player never has to retreat."
This. Having a companion shouldn't be a free boost to your DPS and an immortal meat shield with no drawbacks. Whenever I took on a companion in Oblivion, Fallout 3, etc, I was accepting a double edged sword; on the plus side, you're essentially doubling your damage and halving the amount of damage your character takes; on the down side, you're essentially accepting that there are now two ways to get sent back to last checkpoint; if you or if your companion dies. Removing all drawbacks to having a companion just seems to make the game way too easy in my opinion. And yes, the obvious counter argument is "well just reload checkpoint whenever they're knocked unconscious"- the issue is that you're no longer being forced to either reload or accept the loss of your companion. I can guarantee you there will be times when I'll have slogged through a hellhole of a dungeon, my companion will go down at the last minute, and I'll go "grah whatever, reloading is going to be too much of a pain." But I'll also feel like I cheated the dungeon, which will ultimately give me less of an enjoyable experience.
As many have already said, the solution to this whole issue is simple; instead of just having a "hardcoe mode,' give us all the different aspect of hardcoe mode as gameplay options, so I can just play with dehydration and companion mortality while my friend plays with just exhaustion and non-regenning health.