Even if you have insurance, you can pretty much lose your balance when coinbase becomes fucked up(closed, and many more scenarios like what others points out). In fact, making sure your account is vulnerable or not is just one way to keep your funds safe from other users, but not to coinbase owners. It is still better to store big funds on a software wallet generated offline or a hardware wallet(like what others suggest) but in my case anything will do so long as you own and control your private key.