His comment answered the 'why not port it?' by mentioning Windows 7. Going to Qt6 prevents it from going with Windows 7.
On one hand, I get it, Windows 7 was the last edition before the platform agenda shifted to be all about cloud accounts, telemetry, and being an ad platform. So if you are a Windows die-hard but can't get on board with that BS, then Windows 7 is it.
On the other hand, Windows 7 is being left behind by Microsoft and a bunch of applications. Chrome has left it behind. Firefox has mostly left it behind, and ESR will finally leave it behind by end of this year. Many games left Windows 7 behind (Vulkan and DirectX 12 are generally non-starters in Windows 7). One music player won't balance out the fact you will not be able to run most new games and can't run new versions of browsers.
So ultimately, it's time to leave Windows 7 behind. If you can't get behind the new Windows, then buy a Mac or run Linux. At this point, Wine on Linux might be able to run a broader number of Windows applications than Windows 7, since it does support DirectX 12 and implements other Windows APIs up through Windows 11.