I don't mean expended, I mean locked out. Like you lose access to them until you take some kind of extended rest with medical care or regeneration or whatever else.So at 3rd level I do a combat, get beat up a good bit, spend my 3 hit die to short rest....and now every crit past that is instant death?
Hit Die are too swingy. If you make them too much of a death buffer requirement, people won't use them for their actual intended purpose....healing.
It still means a critical hit for a low-level character would be pretty much instant death, but honestly a critical hit is almost always instant death for a low-level character!