The reason everyone always cites for going native is "performance", but, tbh, it seems like most web projects don't pay attention to the same level of detail as native apps because the web is so much faster to prototype. I bet if devs spent the same amount of time working on perf and loading states on the web as they did on native, the performance argument would have much less impact.
We tried testing that theory, having one team prototype the same app using web technologies and the other prototyping it using iOS native technologies. iOS appeared to be the faster of the two.
The vast majority of the work was prototypeable in Storyboards directly, and only required, if I recall correctly, around 20 lines of code for supporting requirements to build out the prototype. I haven't found many tools for the web that can compete with that.
Of course it depends on what you are trying to develop. If your application is document-centric, then it will be difficult to beat HTML. That is, after all, what it is designed for. But for an application-centric design, I'm not sure it is really all that fast as a prototyping platform – unless, I suppose, that is all you know. It does take time to ramp up familiarity with new platforms.
Interface Builder is more than just an interface builder, it is a serialized data builder. That distinction means that you can instantiate virtually any class in your project without having to write a line of code. It is not limited to just UI elements. That turns out to be pretty powerful.
Below that level, the deserializer exploits a number of properties of the Objective-C's dynamic runtime in order to provide that instantiation of random objects and properties. There are certainly other languages that can be exploited in the same way, but it is not something that can be done directly in just any language.
Without Objective-C, or at least something like it, there would be no Interface Builder as we know it today.
The test case that I'd like to see is someone replacing notepad.exe with a webapp in such a way that an end user cannot tell that a change has been done. It's the Turing test of usability. Web Apps can do an excellent editor, but the finer details like (associate *.txt, save, save-as etc.) are still a problem.