> Why shouldn't I be the only one with the right to sell that image?

The real answer is why should you? Copyright is not a natural right (if it was, it wouldn't require enforcement actions on people who never interacted with the original copy). It's a privilege meant to provide an incentive for artists (the effectiveness of which is questionable, even without taking into consideration the negative side-effects).

> The course of action for "lose the rights in 20 years" isn't "ok, that's fine" but rather "ok, then I won't publish any of them and deal only in direct sales of prints from a gallery."

And that is entirely within your rights. But once your photo is disseminated into the public sphere, it's information that you no longer have control over.