You've completely misunderstood the social contract inherent in copyright. There's no theft in adaptation. Copyright is an intentional trade-off by society to incentivise the creation of new works for society's benefit by giving authors a temporary monopoly. Perpetual copyright would obviously maximize the incentives for authors, but harm society by precluding the creation of new works based on the original. Instead, society chooses a limited period where authors can get most of the benefit while trying to keep the period short enough that works remain relevant.

Saying it's all theft entirely misses the point.