I agree with pretty much everything you said, but I also agree it doesn't argue against the author. That is to say "why not both?" I remember using NT on a domain with active directory profiles in the 90s, and while my memory is quite foggy, I believe some settings were carried between computers on my profile.
In the world of today, I can lock my taskbar on the side instead of the bottom and set my wallpaper, and the customization is saved to my Microsoft account, and loaded on other computers when I login (unless there's a local override, like Bing Wallpaper.)
But I believe colors (like your Accent Color) is still specific to each device. And I have no idea why - though I don't mind since I like using a different color on my laptop and desktop.
I'm rambling, but I think they could still allow all the customization, but also store parts of it on the cloud and carry it with you.
Not just some, most of it (some sensible settings such as hardware configuration are of course excluded, but if you somehow always want to have a 640x480 resolution then you're on your own). Even correctly-coded applications that can differentiate bewteen the Local (%LocalAppData%) and Roaming (%AppData%) application data do take advantage of it.
The diminishing ownership of ones's computer only makes your data more linked into your device and the device's manufacturer. We are lucky in that desktops are still free enough that this isn't a problem, but there is severe loss of data when one loses mobile devices, for example.
Perhaps privacy centric individuals who disable the built-in backup systems would have a hard time, but the typical user goes with the defaults most of the time.
The downside is that I can't just change machines nilly-willy, I need to sync first, at least if things overlap. For example, I can use one Firefox profile on one machine, another on a second machine, and use Thunderbird on a third, but I can't use the same profile on several machines without getting a merge conflict, so to speak. That was confusing and annoying for like a week, since then it's not been a problem, and by now I'm so pampered by it, I simply avoid stuff that doesn't play nicely with my workflow. Smartphone apps, for example, or programs that doesn't allow me to configure the paths it uses, and so on.
Since I use applications and work on data, the OS doesn't that matter much. I mind changing Windows from the ass-backwards defaults much less than sticking with the defaults, and I do that as I go along, i.e. when I use a feature or get annoyed by the default, I just change it. That doesn't require any thought and very little time, so the configuration I would have to repeat on a new system isn't something I consider valuable, I'm pretty sure it takes much less time in total than working with the default config, in the long run. If I had to do it more often than every once in a blue moon, I'd find ways to automate it.
I am just not interested in something that could go away based on the decision of some pointy-haired boss, which is tends to be inferior software in the first place, anyway. Like, compare Directory Opus with Windows Explorer, one is a serious application, the other is a toy that constantly gets in your way.
Sure, it's not an effect of themes, for sure. It's everything from, code for all my personal projects are in a git repo somewhere online, important personal media are backed up in the cloud or external drives, text chat history are backed up in some app or in the cloud, music is listened to on some app, etc. So much so that if a laptop of mine is bricked randomly one day, I don't suffer much of a loss (other than spending the money to get a new one).
Back in the 90s or early 00s, it would've been catastrophic to lose a computer. I would have had so many personal files of all kinds, customizations of all kinds, that would be lost. Nowadays, a machine is mostly disposable. If one is lost, I simply get a new one, and all my stuff is online or accessible somewhere. I no longer think of my computer as a prized possession I can't lose, but rather just another tool.