The shards have thousands of different validators, all of which are staking their Ethereum on being accurate, and if they are malicious, they lose that Ethereum

The shards have thousands of different validators, all of which are staking their Ethereum on being accurate, and if they are malicious, they lose that Ethereum they staked. There are over 100k people signed up to this already (each staking 32+ ETH). So the likelihood of a shard going offline or forking is about as likely as AWS going down. The beacon chain is the central chain, all shards write a hash of what they've done to this one. All nodes validate the shard they've been assigned to as well as the beacon chain.