For anything to be 'theft' you need to have a concept of ownership in the first place. You seem to want a concept of ownership that exists independent of our current society, but this is unsustainable. Ownership must be defined by the society in which we presently exist. Anyone can claim to own anything they like but there has to be a single entity which makes the ultimate decision on whose claims are true. That is a monopoly, it is true, but nothing else would be remotely practical.