Thanks for the detailed answer!
Yeah, with the new File System plugin the NWjs kind of becomes obsolete. I would switch to WebView2 if it had Steam support.
EDIT: I just realized that File System plugin only has access to "picked" files. This is a big limitation. I haven't seen games which display a system file picker dialogue in ages, it will look very unprofessional!
I'd be interested in thoughts on using Windows WebView2 for Steam publishing, assuming we got it to work - would the lack of macOS or Linux support be significant?
I think it is significant. Large publishers want to cover all platforms, it will be a big problem if the game doesn't work on MacOs. There is also a growing number of Steam Deck players, so Linux support is pretty important.
I'd add it should be possible for third-party developers to step in too
Yeah, I was going to ask - maybe you could team up with someone like Chadori or Mikal to develop a new Steam addon?