Debunking the famous "reward argument"

Putting all the property questions aside and looking at the issue from what some people like to refer to as the "moral standpoint", the most frequently brought up argument is that copyright is the reward to an author who by creating something unique has made a valuable contribution to the culture.

This argument, which some view as the moral maxim which should be obvious to any human being, is, in my view, a grave misunderstanding.

The response is really quite simple.
What copyright does as a reward for a contribution is give the author the power to effectively reduce said contribution to almost zero. Indeed, what copyright does is give an author total control of how other people use his ideas - basically, an author in the modern copyright society has all the tools to prohibit any usage of his art after it has already been published.

A reward the value of which is to allow the benefactor to destroy or reduce the scope of his contribution does not sound as something moral or in fact something that makes any sense.

A contribution to the culture is to write a book that becomes part of the culture - meaning, that people can read it, share it, base books, movies and other forms of art on it, remix it in all possible ways and actually use the ideas. This is what contribution is. By rendering a book read-only an author does very little contribution to the culture of today - his real contribution is to the people of tomorrow, several generations away, provided copyright is not extended.

So not only the idea of reward makes no sense, it really advocates for a regime where current generations will use their finances and freedom to fuel culture which they will not even have access to.

The biggest example for me is Harry Potter. What great cultural things could have been built on top of it! How many movies, books, games could've been made! But the strict copyright situation reduces those possibilities to several authorized and expensive productions, most of them quite poor, I must say. And so Harry Potter today is really left outside of our culture. It is just a read-only book you can't really do much with. It is a book for people of the future.
Will it be relevant to them? We know that it is a book relevant today, which caught the spirit of today. There is no guarantee future generations will care about it or that it will click for them the way it does for us.

So where is the contribution? Why should we be grateful to someone who made a book which might as well be unpublished as culture is concerned? When historians look back, what influence of Harry Potter series will they find? Sales reports?