I take it this is your first market cycle?
If you don’t offload to a private wallet and Coinbase goes completely defunct, there is no protection. Coinbase’s continued existence (and thus, your coins) and the market cycle of cryptocurrencies are mutually exclusive events.
Crypto really is just speed running early banking and financial systems.
Many bank runs would destroy banks and depositors were left with nothing. Eventually the FDIC was created to make sure people wouldn't lose everything and to prevent additional bank runs.
What's also interesting to think about is if we will ever see a coin halt transactions to prevent price changes. BTC is unlikely to face this since it's based on proof of work, but maybe a proof of stake protocol might have something like this happen, or a coin that has a small number of miners.
Maybe it would be similar to the few times NYSE has halted trading: https://www.thestreet.com/markets/history-of-notable-market-...
https://help.coinbase.com/en/coinbase/other-topics/legal-pol...