There are three distinct things, don't mix them up - UTXOs, wallet and identity. A wallet has many UTXOs, an identity can have many wallets (hence many UTXOs). Now, there is no link between the identity and the UTXO. Many people miss this key point which should be emphasised more, that you can't know for sure who controls the UTXO once it's moved out of your control, except for ofcourse when you just received or sent a payment where you know for sure what UTXOs your sender/receiver owns. There are many ways a UTXO can move and morph, and at every stage you can just make assumptions about who might have control over it, not be sure of it. Overtime, i believe many types of transactions will occur, coinjoined, coinswap, etc and will be in the majority and it would become even more hard (impossible rather) tell determine who owns a certain UTXO.