On KDE, something nice is that if you have a maximized window and a panel on the top of the screen, I can drag that panel to grab the window (or maybe it was a setting of Latte dock or something). And since window titlebars nowadays can be cluttered with buttons, it is a predictable way to grab those windows only using the mouse.
That you were more or less forced to adopt these KDE shortcuts so that you could work around the fact that they had cannibalized the title bar for a purpose it was not designed for.
You were forced to change your workflow and everybody else is having to be forced to adapt because they changed a metaphor that has remained stable on the desktop for over 40 years
All of the "positive" items I listed come with drawbacks. I didn't realize I might be in the minority for this one, since I genuinely prefer the new workflow.
Today I have one 3:2 and one portrait monitor so compacted titlebars are particularly poor design.
Thankfully KDE for the most part does not indulge in that, and let’s you fix window borders, but they have other failures such as hard coded button order in dialogs.
Also KDE seems pretty staunchly _against_ client-side decorations with buttons other than the window manager buttons.
UX design is treated as a subjective matter, as if it is equally valid to clearly label UI elements as it is to have magic, nondescript UI pixels that serve as vital control surfaces.
Go watch videos of the research Xerox did on UI/UX and HCI in general, and weep for what we have lost...
My argument for the titlebar is that it was at least researched UI/UX convention done by Apple/IBM/Microsoft at the nascence of personal computing. These are the primitives that arose from that research.
It is not out of what I happen to like that I argue this. I personally am deeply frustrated by cursor-y window controls. I much prefer a tiled interface with a top menu bar and copious keyboard shortcut compositions. I, if I could, would Never use the mouse, and if I needed axial control for a 3d environment would prefer to use an analog stick of some sort. Unfortunately those are not the conventions we have for general computing, especially in the workplace.
We are in general forced to use the conventions given to us by the major OS providers. One of those used to be the titlebar, with which you could use the cursor to control the window. The insistence of the current tech industry to shove any button they like up there without regard for these conventions that have been set for decades (and were in many cases of this sin set by their company: M∫/M$ --the integral symbol is a slop 'S') is causing real economic harm in terms of lost productivity from broken muscle memory and wasted actions
...And sometimes I'm using OpenBSD, so titlebar buttons introduce a titlebar I didn't want, and didn't need, which doesn't match the rest of my desktop customizations.
It's just a bad paradigm.