The App Store should not be the only way to get software on an iPhone, sans literally hacking the device.
I want to Apple to be the benevolent dictator of what software is on my phone, And I am willing to pay higher up prices, and put up with missing features in order to protect their revenue stream in exchange for that.
You could still exercise that preference even if Apple opened its platform to (easy) side-loading or other app-stores: you'd just choose to use only Apple's route for software installation.
If the Apple App Store is only one of many ways to install apps on an iOS device, then there's no issue with Apple charging whatever fees it wants. The problem here is the anti-competitive nature, since all roads lead through that same App Store -- you have to choose another phone entirely to avoid the Apple tax.
You seem to be objecting to the manner in which they compete.
I absolutely am! There is such a thing as unfair competition. As I see it, Apple is using their existing dominance to create barriers to entry for new players.
I'm free to avoid Amazon too, but as increasing amounts of society move to Amazon as the de-facto web store for everything, how reasonable is it to fully boycott them? Sure it's possible, but it would be a substantial life commitment.
The number one phone manufacturer is Samsung, not Apple. Huawei is number two.
Users are not free to use other software on Apple hardware.
The operative question is how we define the market. Is the operative market "all smartphones," or are iOS devices distinct enough to qualify as a market of their own?
The answer isn't clear to me, especially because there are a variety of soft barriers. Even if "all smartphones" is the relevant market, phones are expensive; it takes on the order of $1k to walk away from an iPhone X in favour of an equivalent Android device.