you are assuming the BTC's value reflects Supply Vs Demand, and there for if the economy expands or contarts the value of each BTC MUST change too.
this is false
It might make more sense to think of it this way: value does reflect Supply vs Demand, and Demand can be divided into two sources:
1. demand from people who buy it for the sake of speculation
2. demand from people who buy it to use it for non-speculative purposes (i.e. as a currency)
You can subdivide its purposes further of course if you wish.