This generates a lot of additional questions. What about a multisig situation? Can I have a buddy withhold a digital signature until I want to transact so that I did not own the crypto up until the transaction? Add in multiple jurisdictions of private key holders and things could get really interesting. Maybe it would be possible to pay no taxes at all! What about exchange coins where the person has no private keys and hence owns no crypto at all? Would they still have to pay taxes on crypto they clearly don't possess? What an absolute mess.