Where the GPU has backlight control registers, these will usually work and so there's an incentive to always use them - except for systems where the firmware expects to be able to drive the backlight (for instance, cases where hitting the backlight control buttons triggers system management interrupts that then cause the firmware to write to the same registers, or firmware-controlled ambient light sensors), which then results in the hardware state and the firmware state being out of sync and the brightness jumping about in an unpredictable way. And even on systems where using the GPU registers is the right thing to do, one dual-GPU systems where the iGPU is hooked up to the screen then you need to use its backlight control registers even if you're rendering on the dGPU. And on dual-GPU Apples, you need to ignore the GPU registers because the backlight is controlled by the gmux and hitting the GPU registers can fuck things up in amazingly hilarious ways.
If you ignore all these problems then you end up with a simpler interface, and it probably works for a decent percentage of use cases. The same applies to input (things like libratbag exist to apply a level of abstraction to more complicated mouse-style input devices, which you can ignore if you don't care), and to a whole bunch of things if you don't need to care about multiseat.
(The discussion about security doesn't match my experience of working with the Linux graphics community - patches and even new drivers have been rejected based on their security impact, even if they add new functionality)
 I wrote libbacklight
1) Working generic ACPI backlight control
2) A Thinkpad
3) A Toshiba
4) An Intel GPU
Further, in the case where your machine satisfies more than one of these, you'll end up using whichever one attached last with no way for userland to override it. So, yeah, easier to use but failing to solve a bunch of the complexity as a result.
On OpenBSD, RandR xbacklight(1) often just works out of the box, and as the author of the article writes, involves much less magic than the rituals involving systemd on Linux.
Perhaps your Linux bias is showing little as you attempt to justify the complexity of the interfaces Linux provides.. care to go for a round on epoll(2), a botched attempt at copying kqueue(2)? Or how about getrandom(2), a Linux kitchen sink that sprung out of OpenBSD getentropy(2).
I'm not interested in defending Linux in general, I'm just pushing back against the idea that a low complexity implementation is inherently preferable to a more complex one. All else being equal that's true, but if the additional complexity is associated with additional useful functionality then it's really up to the person making that claim to demonstrate that a less complex implementation could provide the same functionality or to make it clear that they're ok with not satisfying the use cases that require that functionality.
The article even hints at a better solution than what Linux or OpenBSD do: Add backlight control in a away that provides 1:1 (or 1:0) mapping between display devices and backlights by, for example, adding the backlight API to the display device file. A naming convention for device files would also work. So would a directory of device files associated with a given screen.
You could even have a device file that lets you tell the kernel if it should use ACPI or something else, assuming you don’t like the default.
What doesn’t work is the status quo in Linux, where there’s a 50% chance I have to implement a new shell script that futzes with some one-off device file in /sys every time I buy a new laptop. (And then use xbindkeys, or whatever to make the keyboard shortcut work)
Fixing this would require the type of thankless work that never quite gets done in the Linux kernel.
I suspect it is because no one has a financial motivation to fix things like this, but they do have a financial motivation to bring up new hardware and to strong-arm through platform lock in plays like systemd.
edit: and thank you for libbacklight. It works beautifully on the other laptops. :-)
If you're having to write shell scripts then there's absolutely a failure - but I haven't bought a laptop in years where that's been the case (and I stopped doing this stuff professionally a few years ago, so I really do appreciate stuff working out of the box). Desktop environments should be providing the necessary glue to bind keys to something that can make an appropriate decision while still allowing overrides for the corner cases that the kernel can't catch.
Systemd actually makes it easier to handle a lot of these -it provides a centralised mechanism for exposing platform-specific quirks to other userland components. I'm not going defend every systemd design decision, but honestly it's been an improvement for everything discussed in this post.