EIP-4521: ERC-721 / ERC-20-compatible transfer

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The goals of EIP-4521 are modest but important for Ethereum users. Namely, it recommends a transfer() function for ERC-721 ‘NFTs’ in order to achieve backwards compatibility with ERC-20 tokens.

Currently, ERC-721 only supports transferFrom() and safeTransferFrom(), and in the event that an NFT is accidentally sent to a contract that only expects ERC-20, it will be locked and unable to be transferred out again.

Further, it is a convenience to smart contract developers to be able to write their contracts in the ERC-20 interface and yet have the ability to manage NFTs at same time without the need to import independent logic. Thus, EIP-4521 suggests a basic extension that allows tokens to work better with user apps, like wallets, and developers who want simpler code to handle a variety of assets, by placing tokenId into the typical transfer function:

function transfer(address to, uint256 tokenId) public returns (bool success) { ... )

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Nice. I see this makes ERC721 contracts more interoperable with software that works with the ERC20 standard.

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Can you help me understand this EIP better and provide an example where same logic can be applied to fungible and non-fungible tokens in a way that amount and id are interchangeable?

Even though the transfer function will not fail, I see a bunch of use cases where contract handles some kind of logic around token amount, which cannot be substituted by id.