That's a fear I have sometimes too. But coinbase is not some scam company. It has reputed and known founders. But still, if you want to keep your coins safe, store your coins in a hardware wallet.
That's the same thing as "hide your money under your pillow", what's the point? isn't a bank account better?
Crypto Currency is just a bullshit a few people made to scam the people's money out.
It's only useful for pirates, smugglers, thieves, drug dealers, scammers, who don't want their money to be traceable so that they can skip the money laundry.