"I want other people to have less choice"
I do feel that a lot of this kind of arguments can be distilled into individual vs collective rights. Having more choice both be good for you as an individual, while also being bad for people as a collective.
For example, a small number of people may argue for the right to install whatever apps they want, believing that they are technologically-savvy enough to manage the risks. They are not wrong, and they are also not the ones who have to manage any fallout at the end of the day.
The problem then comes when everyone has access to the same options, you can't guarantee that everyone making use of said option are cognisant the inherent risks involved, and it's Apple who has to end up managing the aggregated risks to their platform. For example, I don't have to install a third party App Store, but who knows what other security vulnerabilities or tradeoffs may be introduced to the OS in order to accommodate said feature, which impacts all iOS users universally regardless of whether they make use of said feature or not.
You claim there won't be any disadvantage to me at all, but again, how certain are you? I am pretty sure people here will say anything in order to win an argument; doesn't mean that argument will hold up in the long run.
And of course, this argument could just as easily around be turned on its head. You can argue that perhaps it is in the collective interests of Apple users in general that they be allowed to sideload whatever apps they want (like in the case of people in Russia who are not able to access VPN apps overnight), and that perhaps my desire for a safer and more secure computing environment is the exception, rather than the norm.
It really comes down to whether the people desiring third party apps stores and sideloading is the majority or (I believe) the overwhelming minority, and whose perspective we are looking from.
It's a similar argument regarding the repairability of Apple devices. Yes, at an individual level, if and when your iPhone or iPad breaks down, it would be convenient to be able to bring it to the nearest mobile repair shop and have the screen or battery replaced for less money compared to making an appointment with the Apple Store (which is actually not that near my house). But is making their devices more modular and serviceable just for the convenience of the 0.1% worth the global increase in cost, energy use, pollution etc for everyone else who uses Apple devices? I actually don't mind paying more, and making a trip down to the Apple Store for what is probably a once-very-few-years occurance. I will just treat it as an opportunity to do some window shopping while I am at it.
From this perspective, I can understand why Apple chooses to replace a 2018 iPad Pro instead of attempting to service the battery, if they are confident enough of the durability and longevity of their devices and believe that the projected low repair rates make it cheaper to simply swap out the entire device than keep spare parts around (which entails costs of its own). Again, sucks for the minority (especially if they live in countries without a strong Apple retail presence), but easier for Apple to manage from their end.
And even if it does have the ramification of shutting out businesses like ifixit or independent repair shops, one can also argue that it's not really Apple's responsibly to prop up these enterprises.
Or to cite a more controversial topic - US gun laws. It's possible that owning a gun can make an individual safer (or at least feel safer), at the expense of the community being more unsafe as a result.
I guess where I am going with this is that while Apple may not be entirely right with their actions, it doesn't mean their viewpoint (or our concerns) are entirely invalid either (and I will still argue that they are acting in good faith).
Make of it what you will. 😬