Apart from the consistent GUI layer, I think an underrated reason that many teams stick with Electron is the mature tooling for cross-platform builds and upgrades. It's pretty painful to DIY.
It looks like NodeGUI doesn't currently support cross-compilation--is that something that's on the roadmap? How about upgrade/auto-upgrade tooling? Code signing?
This is a super-interesting project though.
Agree with your final part, very exciting and gonna be interesting to see where nodegui goes, especially security wise.
You also need to need to redo your subscription/payments to support the stores and give a percentage of your revenue to Apple/Microsoft. On mobile you have no option but on desktop it's hard to justify giving up so much of your profit margin when there's a popular alternative.
BTW, Linux has the Snap Store.
That said, I am writing a Git desktop client in Flutter and I like it. But I don't expect to finish it for many years so hopefully by then they'll have ironed everything out.
That's already GTK+ and Qt, and the big GTK+ and Qt projects aren't switching over to Flutter any time soon.