Capital gains are paid on cash, not on unrealized gains against unsold assets. So someone doing taxes this year either sold at a profit and has gains to pay on

Capital gains are paid on cash, not on unrealized gains against unsold assets. So someone doing taxes this year either sold at a profit and has gains to pay on cash, sold at a loss and has no tax to pay, or bought and held, again, no taxes to pay. There's no being "on the hook for last year's BTC prices" in any of these.