I'd argue that at this point, if you're building a desktop application, one of your goals is probably going to be the use of native widgets, which could be for e.g. accessibility concerns. Else you might as well build a web application.
The reasons to use something like Qt instead are many:
- While the widgets aren't platform-native, they emulate the native look and feel, so look a lot more native than your web application. Yes, yes, you likely won't have the accessibility features, so if that's a concern (which it probably should be), sure, Qt and other such libraries won't help. (Note that even MS don't use their own "native" widgets in, eg, Office and Visual Studio, so even within first-party windows applications, you lose consistent look and feel)
- Battery life. A C++ Qt application typically uses much less battery than a similar web application. This may or may not be a concern.
- Smooth animation. QML's OpenGL-based widget rendering has incredibly smooth animation support (and its super easy to add to your applications). Anecdotally, much nicer animations than I've ever seen in web applications.
- Platform access or native libraries. Even though your widgets aren't native widgets, you might still want to manually access platform features or C/native libraries. While some web-applications-in-a-webview tools do allow you to do this, Qt makes it trivial since you can drop down to C++ with ease. With conditional compilation, you can even make this work cross platform.
- Performance. Sometimes you have requirements that simply require C++ performance. Many industrial users of Qt, for example.
- Qt is also used for embedded UI's where a web application may not be feasible.
My point is that there are many reasons why you might choose to use a toolkit like Qt for desktop applications even if you don't need native widgets.
