It’s not being stolen because he would have published it knowing the copyright laws. Also even with copyright laws as they are, selling unlicensed copies isn’t theft. It’s illegal but no stealing is involved.
I'd argue that the question is pretty much what should constitute "stealing" and what doesn't. You're certainly entitled to the opinion that it is, but that's a bit circular in terms of justifying a length of copyright. Not everyone will agree with you on whether it would make sense to consider it "stealing" after a certain length of time.
Looking past that specific word choice, there's an implication here that only the author would have an unbiased opinion on it. I'd argue that they're just as likely to have a bias that would cause them to argue for a policy that is unnecessarily onerous because by the same logic, they're not the ones who would be missing or on anything from it.
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.
