If the iPhone was an open platform, you could develop a mod that makes your Fortnite character look like whatever you want, for free. (It wouldn't show up that way for other people by default, but that's someone else's phone, not yours.)

Correct me if I'm wrong here, but I'd expect that iOS being an open platform is neither necessary nor sufficient for that. As I understand it, Epic could allow users to load free custom skins today, even though it operates on a closed platform. And conversely, even if iOS were an open platform, Fortnite could remain closed-source, and I'd expect Epic could take measures (hash-validating assets on launch?) to prevent side-loading of cosmetic items.

On a traditional PC, or on a Mac with System Integrity Protection disabled, there isn't a lot Epic could do to prevent installing mods. The user can inject whatever code they like into the Fortnite process. They can rewrite data in memory, and even control the kernel if it comes to that.

Epic could try to make life difficult for modders, but a lot of PC games have tried that over the years, and it generally doesn't work when players are dedicated.

The content creator selects which currencies they are willing to sell their product for. This shouldn't be difficult to understand. Do I have the right to demand that my local furniture store accept Bitcoin?

Fortnite is a free game, I pay to own my iPhone.

Should it be different for similar games that aren’t free (gratis) then? Should I be required by law to be able to load third-party items in Call of Duty?

There needs to be different rules for products that are essential to modern life, like smartphones.

We have more rules and regulations for things like food services, airlines, banking and farming than we do for a store like GameStop or a jewelry products.

Then if you care so much about running whatever software you want on it, perhaps you should have bought a phone that lets you do that instead?

I'm not personally in favor of walled gardens myself, but everyone who bought an iPhone knew that's what they were getting and did it anyway.

"everyone who bought an iPhone knew that's what they were getting and did it anyway" is an incredibly strong assertion you're not making an effort to support here. Are you seriously claiming that every owner of an iPhone understood the nuances of Apple's app store policies before they bought the phone?

Everyone in the world? No, clearly, but I doubt very many of them care about being able to use other stores on their phone anyway. The people here making the argument for forcing Apple's hand? Yes.

People pay to own their tractors, but yet they can't repair them.

Yes, and that's not right.