What is it exactly that we're rushing towards?

[Psyren]

So, if I have this right, Psyren, it sounds like you're saying Luck and Meat are essentially intermixed. That even if you're at 100% HP, a poisoned arrow that hits higher than your AC is finding a bit of meat, dealing poison HP damage to that meat, but you still have Luck HP hanging out ready to "avoid" a sword swing (provided said poison doesn't drop you to 0 HP).

I can get behind that interpretation for a 5E game.

If I misconstrued your point, I apologize.

You're pretty much there. The key is that meat != meat, even in our world - a stingray stabbing your ankle and one stabbing your heart are both hitting "meat" but one is a lot more likely to kill you, and in the HP abstraction the one that got to your heart either got you while you were already worn down (low avoidance/luck) from something else, or you got supremely unlucky (critical hit, which bypassed a lot of your "luck" entirely.) Or even both. Either way, 5e would represent the latter scenario by the blow doing enough damage to take you to zero.

I think this is about the only way single pool HP can works regardless of the system. IMO for DND it's probably the best idea because HP is practically an unavoidable degrees of progress to prevent instan-splat both ways.

I think it works better if you can't just easily fill up between or in fights but that's a personal taste thing.

To me, the far easier way to address this (if it even needs addressing) is by implementing one of the grittier rest variants, rather than by trying to overhaul/bifurcate HP entirely.