In my experience it's not that simple. I certainly don't believe Krita written in Java Swing would be as successful.
There's a lot of complexity in GUI frameworks, and they are not interchangeable because they end up making different design choices. An application like Maya with very complex user-manipulated data structures will expose weaknesses in the framework, and the fixes and design improvements end up in the framework. A competing framework whose primary users are lightweight consumer-oriented apps doesn't get those benefits.
> In my experience it's not that simple. I certainly don't believe Krita written in Java Swing would be as successful.
I disagree here, i'm certain it would be as successful because the GUI framework is not the reason for Krita's success, it is the functionality it provides and how the developers interact with the community. The GUI framework does not have any image manipulation specific functionality (all of that is implemented by the Krita developers) and the community interaction isn't even a technical thing in the first place.
> There's a lot of complexity in GUI frameworks, and they are not interchangeable because they end up making different design choices.
I did not claim that they are interchangeable (though they can be, depending on the program's design), i even explicitly wrote that taking Qt out of Krita would mean almost rewriting the entire program as it relies heavily on it.
What i claimed was that Qt is not the reason for Krita's success and it could have the same success with other mature toolkits. There is nothing special about Qt aside from being around for long enough time to have its functionality "battle tested". This is not unique to Qt though.
> An application like Maya with very complex user-manipulated data structures will expose weaknesses in the framework, and the fixes and design improvements end up in the framework.
This is the case with any toolkit or really any library that has a lot of applications written against it, assuming the developers do not ignore all bug reports and issues the users of their libraries report.
Also since you brought up Maya specifically, Maya used to be based on the Motif toolkit until Maya 2010 (it was changed to Qt in Maya 2011), which by the same logic would mean that up until 2011, using Motif would be great for professional content creation applications since Maya used it too.
There's a lot of complexity in GUI frameworks, and they are not interchangeable because they end up making different design choices. An application like Maya with very complex user-manipulated data structures will expose weaknesses in the framework, and the fixes and design improvements end up in the framework. A competing framework whose primary users are lightweight consumer-oriented apps doesn't get those benefits.