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?
You're describing an exit scam. Many wallets and exchanges have exit scammed. This is why you should never use exchanges as wallets, or use hosted/web wallets. The old saying goes, "If you don't control the private keys, you don't own the bitcoins."
Coinbase is probably the least likely to cut and run. They're really transparent and hold licenses where they operate. But they could always get hacked or otherwise compromised and lose customer funds that way. If that happens -- worst comes to worst -- they declare bankruptcy and leave things to the courts.
Im pretty sure your money is safe with Coinbase, arent they government insured or some shit?
USD in their custody is FDIC-insured. Maybe other fiat money is insured, too. But cryptocurrency definitely isn't.