Why is the mining cost the "fair" value? Also, I have some bitcoins from when it cost $5 to mine a bitcoin. Does that mean that those bitcoins are worth $5?
It is true in nearest term not in too long term.
People are fearful of dumps from people who will receive compensation from Mt. Gox case because they know those victims got their bitcoins at very cheaper price than price in 2023.
Similar story for miners. Different miners have different entries and entry cost to mine Bitcoin so they will have different net profit or net loss at a time. Sometimes I saw whale alerts that very old bitcoins years ago were moved and people felt panic. Maybe it is a same fear as they thought that if a miner from 2011 moved their bitcoins, that can be a plan to sell it on exchanges.
However I am aware that it is part of market manipulation game.
This
Realized price is useful for Bitcoin investors and their DCA plans.