If private keys are proof of ownership then what's the difference between keys generated before vs keys generated after. If both keys are valid then both have then both are proof.
Private keys are not proof of ownership, they are a technical implementation detail. That crypto has been poorly designed so that keys are all people have to prove ownership is a failure of design but doesn't fundamentally change the divisibility of ownership and access. This is easily observed with a shared wallet, for example coinbase's. If an employee stole their keys, I assure you that employee would enjoy a jail yard view for the next lifetime.