But is that due to technology choices, or due to more attention being paid to UX? (and I do believe webapps give more flexiblity in that regard)

I get the feeling that UX and UI designers for web- and mobile apps are just a different breed than their native desktop equivalents. It might just be my Windows bias though, iirc Windows didn't have a strong / great UX guideline until their current one came around - and I haven't used many modern windows apps yet, the ones I do use are stuck in the 90's with their button bars and such.

That is due to the technologies available on the web being easily more suitable for GUI development than older native tools or desktop frameworks. We're in a similar boat, we made WPF apps. Now we're bringing it to the web and Electron. The things you can do now in React, heck i wouldn't even know where to begin in WPF or QT. If you look at modern Electron apps like Atom, VSCode, Discord and so on, the things they do and how flexible they are, this would mean so much hardship and pain in older solutions to the point where it's probably not viable any longer.

Other than declarative vs imperative, please substantiate your post with examples of what isn't possible in WPF or Cocoa.

I have a feeling it's more of an experience / competency issue rather than lacking APIs.

It's the ease of declaring components and structuring complex applications with it. There is nothing in WPF that is comparable to it unfortunately, except Redux.NET maybe, but XAML is a lost case against JSX. You'd have to see it, or work with it. The bottom line is, drastically less code, clean structures, things become simple to do.

Debugging is also something different: https://camo.githubusercontent.com/a0d66cf145fe35cbe5fb34149...

Like hot module reload where you write your app live. You edit one component, everything else stays in place and maintains state. Or time travel with Redux, where each piece of state is inspectable. You roll back or slide through the apps history and see it literally build and deconstruct itself. That's possible because UI is just a function of state. Same state, same UI.

It's never going to be possible to fully separate the factors out. With infinite time and effort by the exact same set of devs, I'm willing to grant that electron apps will be worse than native apps. But that hypothetical circumstance (same set of devs, infinite time and effort) does not exist in reality.