I don't think Coinbase can go bankrupt because users leave the platform. They would go bankrupt because of mismanaged funds in that case. Let's say all users withdraw their fiat currencies and also cryptocurrencies, Coinbase will still have money in their own bank for sure, unless they store their own money on their platform, which is highly unlikely.

Even if 100% users withdraw 100% of their funds, Coinbase takes a transfer fee from all of those withdrawals, so they'll get even more money in their account in that case.