United StatesCoinbase holds about 5% of all bitcoin for clients.

Coinbase’s blog post says bitcoin should prepare for quantum attacks now, not wait.

Anyone can see a wallet’s public key, and a future quantum computer could use it to recover the private key.

Coinbase wants the dispute over dormant and late-migrating wallets settled separately so it does not delay protections.

How each outlet framed it
CoinDesk
explains the bitcoin lock-and-key mechanism with a mailbox analogy before the threat is stated, and notes Coinbase holds about 5% of all bitcoin

Sources: CoinDesk