Congress did not intend fair use to be an affirmative defense. It is clearly a right under US copyright law. The rights granted to copyright owners in section 106 are expressly “subject to” the fair use defense. The fair use section of the Act, section 107, provides expressly that fair use “is not an infringement of copyright", rather than an infringement with an affirmative defense. In Lenz v. Universal the courts say that fair use is "distinct from affirmative defenses where a use infringes a copyright, but there is no liability due to a valid excuse.” and that “fair use is ‘authorized by the law’ and a copyright holder must consider the existence of fair use before sending a takedown notification….”

It might not be an "affirmative" defense, but to my understanding that doesn't imply it's a right. A "right" as I understand it is something that is illegal for others to violate, and that government generally has a duty to protect. But there's nothing unlawful about creating something that's uncopyable, or about guarding what you have so closely that others are unable to copy it; in fact, I would've assumed being able to do so is itself your right. And should you create something that's uncopyable, nobody will be able to copy it, even if it would be fair use for them to do so. They might be upset about this, but their rights definitely aren't being violated by your creation or protection of that thing—and the government can't force you to make your stuff copyable. (Or maybe it can with additional legislation, but I'm pretty sure we don't have such legislation.) That means "fair use" isn't anyone's right... it's just a valid defense (whether "affirmative" or otherwise).

By this logic free speech isn't a right because the government is under no obligation to provide you with a platform to exercise it.

Offering you a platform is not the same thing as ensuring that it's even physically possible. The argument wasn't about the government hosting copyrightable content for you or delivering it to you; it was about ensuring it is even possible for you to copy content.

Preventing something from being copyable is only possible by not distributing it. If you can see it you can copy it.

The copyright holder could prevent you from copying it by never distributing it, but that doesn't mean you don't have a right to fair use, it only means you don't have the ability to exercise that right. Much the same as you can't exercise freedom of the press if you can't afford a printing press (or any modern equivalent).

It might be physically impossible for you to have an abortion, e.g. because you're infertile, but that doesn't mean you don't have a right to one under existing precedent.

Fair use is a right, and if the law doesn't accurately reflect that, then the law needs to be changed.

When a "right" is spoken of in legal discussions (which this clearly is), it implies a "right by law". If you want to talk about another right, like your "moral right", you'd usually say exactly that.

But yes, even rights defined in the law can and do get redefined, newly introduced or removed.