You're both right, but that is not a valid point. KDE is only one of possible system gfx environments - what about dozen of people using other desktops?
There is a reason that chrome or vivaldi introduced such feature in their browsers - they (tend to) care about all of their users not only about the bigger group. The problem is right there, whether you like it or not. Even if I will change desktop, there is sitll big group of people who will reject opera for lack of this feature.
Atm I am using Cinnamon desktop for various reasons and my reluctance to Chrome / Vivaldi isn't big enough to swtich whole desktop just for a browser.