let's assume it costs $4k to mine a bitcoin. If the price were to fall to $10 or some extreme low value, the miners would see no reason to mine. Therefore all activity would stop. But in order for existing bitcoin holders and merchants to use bitcoin, they need transactions confirmed. So this will lead to the price rising to an equilibrium where the miners will mine at a loss and merchants and hodlers will sell at a loss which is optimal for both than if the price is too low.