> These txids are immaterial to how the Bitcoin blockchain works: their primary use is as a convenience for humans when referring to transactions.

This is incorrect. Each Bitcoin transaction input references a previous transaction output as the txid+output index. Transactions spending unconfirmed outputs are orphaned when the parent is malleated and confirmed.

Also, as a data hash with no checksum, txids are not convenient for humans at all.

> Transaction malleability is already more or less fixed in Bitcoin

A couple months ago, there was a significant malleability attack on the Testnet, in which nearly every transaction was malleated as it was included in a block.