I was a moderator on the Paint.NET forums for a long while in the mid to late 00s. You would be surprised at how many questions we got about when Paint.NET would get "the new ribbon UI!"
I don't think so. He just said that he had evaluated it and it wasn't a good fit for the application. I remembered it was in our Popular Feature Requests thread, and I looked back and (crazy enough) it's still there.
I've sort of been forced over to Mac (not that it's a bad thing, just a thing), and Paint.NET is perhaps my biggest loss in that transition. I've loved that program since its early days, and is always one of my first installs on any new Windows installation.
Yeah, I've been trying to make a switch over to Linux for a lot of reasons, but honestly Paint.NET is the one thing that keeps me tethered to Windows that I'm not super grumpy about (Adobe also keeps me tethered to Windows, but that makes me angry every time I think about it).
If *Nix has a decent image editor with layers that isn't super over-engineered like GIMP is, I haven't heard of it yet. Maybe that's all become web-based.