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Well ser, you are wrong. Monetary_premium / store_of_value aspect of money IS zero sum. Ever heard Saylor/others saying "Bitcoin will demonetize other assets", or "Bitcoin will eat the world"? Those refer to this zero sum aspect of monetary premiums.
I find that too frequently, people take the perspective that bitcoin can only succeed at the destruction of value elsewhere in the economy
Okay, perhaps this is where the confusion lie. Bitcoin doesn't destroy "value". If we switch to Bitcoin Standard overnight there is the same amount of cars/houses/stuff in economy. Bitcoin absorbs value from other stores-of-value. The zero-sum aspect only refers to monetary premiums.
If Alice and Bob agree that the purchasing power of 1 BTC is greater than it was yesterday
Bitcoin is pure money. It doesn't have industrial uses (like gold). Value of bitcoin is 100% monetary premium. And as such if Alice is rational actor she doesn't 'decide' to value bitcoin at 2x next day. Her 'decision' is driven by opportunity cost of 'what can I get in other places for 1 BTC', and that is driven by the whole economy.