Electron is not superior. Web grew because of the great "application delivery" system and despite everything else that originated from the static web. By having huge number of js developers the thing was forced to do other tasks. Now with Electron it may as well do desktop apps (by removing the awesome "application delivery" system; isn't that funny?). Its only advantage is that you've got lots of js developers.
The "new" js desktop technology is new only in the sense that js is new to the desktop (so it still doesn't really know about all the desktop requirements). Same for servers: there are better tools for the job from the purely technical point of view.
So is slack, which (from what I've heard, I have no personal experience) eats RAM like a maniac. Neither of these example applications tips the scale between Qt or Electron on being superior.
Again, based on what I've heard/read, Qt being less recourse hungry and written in C++ allows it to be (for example) embedded (to cars or iot devices, for example).
Why Electron is superior? We do mobility simulation and the UI part is the minor of our concerns. We are focused in modelling capabilities and performance (so C++). Having a solution that uses the same language across all the application layers help us a lot.
Because Electron is not "superior". Both have their strengths and weaknesses, and which one is the right choice depends on what you're doing and what your existing skills and components are.
Yes. HTML + DOM + CSS + JS was not created to write interactive applications, and it still shows. Most of the inconvenience can now be worked around with frameworks, but a large performance penalty remains. And Moore's Law is over.
The "new" js desktop technology is new only in the sense that js is new to the desktop (so it still doesn't really know about all the desktop requirements). Same for servers: there are better tools for the job from the purely technical point of view.