You're making the sunk cost fallacy. The amount you paid for an asset is irrelevant to its present value, and equally irrelevant to whether and how much of it to sell.
It does not imply anything about reversibility. It applies just as well to a liquid asset like bitcoin or stock as to an illiquid asset such as a house.
Two people each have 1 BTC. One paid $5 for it, the other paid $10 for it. Their present situations are identical, therefore the rational behavior is the same for both of them, despite their different purchase prices.
How is this a sunk cost fallacy? As an investment, selling off a portion locks in gains, rather than leaving it all at risk should BTC lose significant value.
Selling off a portion of your investment to protect yourself from excessive risk is perfectly rational. Doing it partially as a function of the price you originally paid is not.
