Only the Qt thing is current, but memory is long on these things and a lot of people formed their opinions a decade ago. First impressions and whatnot.
I have not tried KDE in this decade, or the last. The article makes me curious. But Qubes is an Xfce desktop, and I am stuck there for the time being. It has been OK. Qubes desktop can't use a GPU anyway.
If Qt goes proprietary, it will fork without drama.
I don't know what the situation is these days though but anecdotally it seems GNOME has now become the "bloated" desktop, thanks to Javascript in the shell.
Firefox or Chrome will absolutely dwarf your DE in memory usage, so I don't think it's really a major differentiator between KDE and Gnome. If your PC has less than 2GB of memory, you'll run into other issues first.
KDE 4 was a low point, at least the first releases were buggy and didn't feel nearly as good as 3, but 5 has been absolute smooth sailing. It's surprisingly light and nimble, I use it on my Thinkpad X220i and it runs great, better than Cinnamon on Mint did when I tried that out.
There seems to be a lot of friction against giving KDE a go, and I just don't understand it.