In the future there will be billions of dollars stored in Wallets, what if one day they decide to stop and shut down their services and transfer all the money to their own wallet and confirm the transactions and convert all of their coins(Bitcoin and any other Altcoins) to USD and disappear?
How can we get our money back? whom can we sue? how can we prove how much coins we had in their wallet?
That's why it not advisable to store your coins in a wallet wherein you don't hold your own private key. As long as they have access to your coins then it is considered not totally yours. As what others say, it is not your coin if you don't hold the private key. Just store your coins in a hardware wallet or paper wallet as long as you hold the private key then it is your coin and it is safe. The same goes with storing your coin in exchanges. When the exchange decided to shut down then they will bring your coins as well so it would be considered as lost already. If there is no rule yet about cryptocurrencies then you cannot sue anyone.