With the parent_beacon_block_root hashed into the block_hash, there is an on-chain commitment that is exposed to EVM via the blockhash opcode. Hence, contracts have everything they need to validate the parent block root via calldata by simply pushing in the full block header or a matching zk proof.
Not sure if the system contract should be included without it being absolutely necessary, or before there being applications making use of it. The contract just feels like adding relatively heavy infrastructure that will be hard to modify in the future.