I just started prototyping a mobile version of my app (which shares the code as my desktop app) and the result looks promising (still work-in-progress tho).
Offering a web app is indeed not trivial. Maybe Qt WebAssembly will be a viable option if I can optimize the binary and users wouldn't mind first long load time (and then the app should be cached for instant load). Or maybe I could build a read-only web app using web technology.
Currently, my focus is building a good native application, and I think most of my users care about that. But in the future, I can see how a web app could be useful for more users. One thing I would like to built is a web browser that could load both QML and HTML files (using regular web engine), so I could simply deploy my app by serving my QML files without the binary over the internet.
Many such applications are accessible on the web, often with the exact UI. They may even have a mobile/iPad version. They may be big enough that they have a design system that needs to be applied to in every UI (including company website). Building C++ code on all platforms and running all the tests may be too expensive. The list goes on.