Yes you could fork Atom with no native menubar, and you’d have a terrible app on most platforms.
Denial of platform UX/UI conventions just because is not really productive, but rather counterproductive for the users and the app.
Indeed not beholden to, but unless the deltas are really better, you end up increasing cognitive dissonance in users without a return for it.
Although Vim and Emacs are indeed fabulous, they are so in spite of the given platform they run on. Emacs because it truly outdates them all. Eclipse suffers a lot from the same issues in the Java way of being built under AWT and Swing in spite of resulting in a substandard UX much of the time.
Atom has a lot of that due to its Electron roots, but generally does overcome it. Real stinkers are usually around keyboard shortcut soup. Shortcuts are kind of subtly very hard to come up with good ones, but good ones tend to be platform consistent. That’s a similar spot where Emacs and Vim fall over. Mac users learn keyboard shortcuts more than most and the control key would be last if the fn wasn’t there. Emacs keys are based on Lisp machines’ keyboards, whose key layouts resembled the modifier key layout on US English Mac layouts. Classic Emacs shortcuts on modern machines is pretty brutal in many cases.
But if you need to use Vim/vi or Emacs on multiple platforms frequently, because they’re often there by default, you learn and roll with the defaults.
Vim has a notoriously large learning curve, often worse than Emacs due to the utter sea of complexity.
Eclipse, for what it’s worth does try hard to be platform consistent within the confines of what its frameworks would let. Cross platform tools and frameworks often do miss little things that matter. I have filed many bugs for them. JetBrains fixes a lot of what’s wrong there, but not when it is AWT/Swing at fault.
Tcl/Tk although it ships with Macs and always has, is more like a looks-like than an acts like.
Qt probably does the best of crossplatform frameworks to work fairly natively everywhere, but still feels odd much of the time.
You see, these issues, they’re fundamentally what’s wrong with most of the web. It is improving but only thanks to major investment in UI frameworks.