ah that's fair. I am massively OCD about the design of my desktop. I spend hours and hours tweaking pixels and font rendering just to get the drop shadows on the letter 't' looking correct lol.
I haven't written anything in GTK since 2.x days (looks like we're all the way up to 4.x now). I've never written anything for kwin nor worked with it directly before to be fair so I have no idea on that front. I've definitely heard the KDE/Gnome as platforms approach before and I honestly disagree as even if it might be the truth, it shouldn't be in my opinion. Applications should be unified across all desktops - issue is there's 2 fronts to this idea: 1) Devs should make their app look consistent down to window controls across all desktops, or, 2) Devs should make their app look consistent across all desktops but leave drawing of the window controls up to the Desktop Environment (however it may do so).
I used to firmly be in camp 1, but I've shifted over the years to camp 2. What I've never understood is why both seems to be such an impossible compromise. As you said, you can currently just change a dconf key to say the user doesn't want any window controls. It should be that there's a dconf/gconf key to say whether the application prefers to use SSD or CSD. The argument I heard against that back in the day was that it was "too time intensive to maintain two separate implementations", however I have always been of the opinion that that was a cheap and lazy way out of the discussion.
Anyways - sorry for the tangent. I forgot about all of that until just now lol. Was like circa 2011? 2012? Somewhere around in there that all of that transpired for me. I had hoped we were long beyond it but I suppose some arguments are just too silly to expire.