I might not want that choice. That requires that I use my brain as I use the device and not do the harmful thing. Or maybe I want to hand the device to my child and be assured that she cannot install software, or use it as a publicly-accessible kiosk and be assured members of the public can’t break it. People who have handed Windows PCs to software illiterates and have to constantly return to eradicate crapware understand this problem.

Also, it costs the vendor to implement support for installing other software - resources the vendor could have spent on features I value, rather than features I don’t want. If only a government didn’t dictate to the vendor what it should do, stripping the vendor and the user of the power to decide for themselves.

If you don't want that choice you don't have to enable the feature. Installation from Unknown Sources in Android is off by default and requires a user to explicitly go into the settings and find the obscure toggle, which in recent times has proven to be a big enough deterrent that there don't appear to be any recent mass-scale attacks using this vector. On top of that Android and Windows both have fairly comprehensive parental controls, which can disable the entire option of installing non-approved software, so that point is moot.

And the resources needed to make an app installer are not nearly as high as you make it out to be because iOS already has the mechanism to install signed .ipas. All that's needed in theory is a check to disable signing (which they already have implemented in MacOS) and to add a few pages to the Settings app, which surely shouldn't be an issue for a tech company of Apple's size. And if you argue that it might break some spaghetti code then maybe that should be fixed anyways and it's doing them a favour.

Having a choice will not harm you. The "brainpower" required to stick to one appstores is virtually nil, evidenced by the vast majority of android users sticking with only the play store. You don't want other people to have that choice because you're a sycophant for a corporation.

So if Android already offers the choice you seek, why does government need to make Apple offer that choice?

Because there are already users with Apple devices and those users might have been locked into using Apple's ecosystem for other reasons (already purchased apps, already owning Apple accessories that don't work with other devices, etc).

As an anecdote myself, the main reason I haven't switched to a Galaxy S24 is because my Airpods work amazingly with my iPhone and Macbook, and my Apple Watch only works with iPhones. But very often I sorely miss having Termux, NewPipe, Tachiyomi, a non-gimped version of GBoard, Syncthing, a sensible launcher, and probably other things that I can't remember off the top of my head. I've decided that I value the Apple system more than the value I get from those apps but this regulation means I get to have my cake and eat it too.

Because Apple is not necessarily entitled to an anticompetitive market just to provide product differentiation.

> That requires that I use my brain

There, you fell into a trap of your own making.