How does Bitcoin work on the principle that value spent is value stored?
Currency is certainly subjective. I can buy much more with a dollar in Mexico than I can in Switzerland, and a dollar bought more in 1913 than it does today. That's because people assess the value differently depending on their situation and knowledge.
If you think that currency is not subjective, then try to tell me an evaluation of a dollar, or an ounce of silver, or any other currency, which does not change depending on time or place.
You mine for it, but that doesn't mean it relies on the idea that value spent is value stored. That doesn't follow. Precious metals have value because people see them as valuable, and it's the same with Bitcoin.
I'm willing to consider what you're saying, but I find it very difficult to imagine what you're describing.
A good currency should have stability, yes, and bitcoin has gained more stability over time. However, it would be very difficult for a currency to retain exactly the same amount of value, with one pound always and everywhere buying 10 loaves of bread, ten pounds always buying a ladder or something like that. Is that the kind of thing you're describing, or have I misunderstood.
I have no problem with currency being both a measure of value and a store of value, though obviously it's an imperfect measure of value because of the factors I've mentioned. I'd say all forms of money are commodities in some sense.