So black rock is using Coinbase to custody the bitcoin. What if Coinbase loses access to these bitcoins?

Is it possible for them to lose access? How fragile is the wallet or wallets that hold the coins? I assume it’s multi sig. Hopefully they use multiple wallets. If it’s all in one wallet and someone wants to be the most malicious, they could be. I wouldn’t recommend an etf to anyone, because it does not follow what Bitcoin and this subreddit has taught me.