> The ability to create a native app which is at least beautiful as modern WebApps.
I don't consider this desirable or even a reasonable criterion. I want my desktop apps to all use the same toolkit and look consistent. Unless you make a bog-simple app, something like Electron is never going to give you that. With Qt/Gtk/etc. you can certainly make custom widgets if you have special-purpose needs. For someone who comes from a web development background, there's going to be a learning curve for that, of course. But experienced Qt/Gtk developers should not find it meaningfully more difficult.
> e.g. Slack or VS Code (Electron) are beautiful, while fractal or notecase (gtk) are not.
That's incredibly subjective, and I don't agree with it. I think Slack and VS Code are fine looking apps, but I wouldn't rate them meaningfully higher than all the Gtk apps running on my desktop right now. (And regardless, I value consistency over some ill-defined measure of "beauty" when it comes to UI.)