Thanks for your thoughts on this. Don't you think the process of downloading every transaction and verifying them one by one is a tedious and long process. This requires a lot of time. Instead in sharding, the blockchain will divide into seperate shards. Nodes will have to validate the transactions that are assigned to that node only instead of whole ledger.
Aside from cost of decentralization which already mentioned, i doubt regular people would run sharding nodes, unless there's direct reward.
Most people would rather use web wallet (such as MEW & blockchain.com) or SPV wallet/light node (such as geth light node or Electrum).
Besides, on few sharding proposal (such as ETH) mention "super node" which download & verify all shards is still required.
And for security risks, Ethereum has proposed a random sampling concept where authorized person will verify the block authentication. This can eliminate the trustlessness. What do you say?
That means you need to trust the authorized person