Unrelated to market share & OS updates, there's the technical question. There aren't many Win32 APIs which are present in Windows 8, but not Windows 7. Thus it's not that likely that Windows 7 wouldn't be supported by a Win32 app (which this is), unless they go out of their way to make it so.
[1] http://windows.microsoft.com/en-us/windows/lifecycle
[2] http://www.w3schools.com/browsers/browsers_os.asp
[3] https://www.netmarketshare.com/operating-system-market-share...