Your forget one important thing. If a node only hold a shard (part of the blockchain), that means the node couldn't perform full verification. Without reward to run shard node, most people would keep using SPV wallet while those who care about decentralized would simply keep running full node.
If Data Sharding is already on the blockchain then how do we know that it is already implemented? Is there any example of such implementation to relieve the Blockchain Burden?
Few altcoin claim they use sharding to "tackle" scaling problem. But AFAIK only Ethereum being serious about implementing sharding[1].
[1]
https://eips.ethereum.org/EIPS/eip-4844[2]
https://github.com/ethereum/pm/blob/master/Breakout-Room/4844-readiness-checklist.md#client-implementation-status