Bitcoin can't go to zero unless quantum computers break its mining and digital signature cryptography (SHA-256 and ECDSA, respectively) or its blockchain experiences another type of catastrophic failure.

This is also assuming that QC will compromise it before the chain migrates to a quantum-resistant one.

Why? An estimated 3.9 million BTC have been lost due to missing recovery seeds, damaged and discarded hardware, and desktop and mobile non-custodial wallets.

A portion of this will most likely be recovered via specialist recovery services that aim to re-access these BTC wallets. Nonetheless, at least three million won’t be retrieved in Bitcoin’s current form.

These coins cannot be accessed, so they won't be sold off.

You're also missing the point that many people would readily snap up BTC if the market tanks, thus counteracting vast selling pressure.