My argument is that a consumer should be allowed to install whatever software they want on hardware that they own. And those "reasons" have the effect of being anti-competitive, which is a no-no, especially if you're one half of a duopoly. I don't think Apple originally walled off the iPhone from outside apps for money, but that absolutely is a primary reason for maintaining that wall.
In both cases a contributing factor is simple inertia. From the early days of computing and PCs, a walled off garden wasn't a thing. When it came to the iPhone though, especially early on, a walled off app store kind of made sense. It wasn't conceived as a truly stand-alone device as it exists nowadays. It depended on a connected computer for a lot of things, even OS updates. Not being a standalone device meant that they needed an easy way to get apps onto the phone and an app store was an easy way to do that. And Apple giving developers the tools to get their software into the app store meant there would be lots of apps and quickly. At some point though the app store became extremely profitable for Apple and is a primary driver for keeping it that way. No doubt Apple should continue to benefit form the tools they created, but clearly the status quo is far too favorable to Apple.