While i agree that using Project Centennial to convert an existing app to Windows Store app is very simple, even doing it manually. A program as feature rich as paint.net I feel like would be awesome to be re-written in UWP. Obviously this would be a challenging project. The automatic updating feature that paint.net currently has would seemingly be antiquated if the developer would go to the Store app idea. Which seems like it could be a bit of work to strip out of the program. Like I've stated before, I'm no app developer.
I have been quietly been lurking around the forums for years never posting, in fact i just made my account to share my $0.02!
Keep up the great work Rick and Team.
Cheers!