I agree—that’s why I’m not proposing a UTXO-set commitment.
Instead, the spender provides the usual SPV proof (Merkle inclusion of the transaction in its block), which you simply check against the header chain.
So the blockchain itself acts as a TXO commitment.
The only new data structure is a lightweight list of 8-byte “locations”, representing the unspent expired UTXOs:
location = block_height / tx_index / output_index