What's immersion have to do with being put at an (unrealistic) mechanical disadvantage?
The easy answer to this is for me to ask you to quantify the current number of HP that you have IRL at this moment and your maximum HP. Also, can you quantify in real time the amount of HP that you lose from a combat situation?
The average man has a Str of 13, and swings for 1d4 points of damage per strike. In fights before, I have taken about five or six or so solid blows before getting to the point where I could no longer fight (or in otherwise, am staggered). The average damage per blow is going to be 3.5 damage (2.5+1 for strength), therefore I possess about 15 to 18 hit points, at least as far as nonlethal damage goes.
I am at the peak of my condition for right now (no injuries, sprains, drunkeness penalizing my Constitution, nor illnesses). Therefore, I possess from 15 to 18 hit points.
Without being in a combat situation, I cannot answer the second part of the question.
Which is about accurate, saying I'm a 2nd or 3rd level expert with the constitution of a horse is an adequate statement in regards to my abilities, but this isn't really the thread for it.
Also yes, hit points have always been will to keep on fighting/ability to prevent a mortal or killing blow; even the 1e AD&D DMG says 'hit points are necessary abstraction; a high level Fighter can have more hit points than two heavy warhorses. It isn't that the Fighter can simply take continual flesh-wounds from orcish battleaxes, but that the fighter has the ability to roll to avoid all the damage, or deflect it to his armor, or his shield; his ability to withstand damage and stave-off the final blow is lessened when he takes 'damage', perhaps reflecting his shield arm being numbed and unresponsive, or him being winded'.
Or something very close to that.