>wxWidgets

This is, in theory, what you're asking for. It's not perfect. But nobody uses it and there are few contributors. In most threads lamenting Electron, it barely gets mentioned. So:

>I really don't see any massive barrier

I think the barrier is a lack of interest. If anyone thinks about building a cross-platform native-widget GUI framework, they only have to consider briefly the near-total obscurity of wxWidgets and how much work it would take (a lot!) to even reach feature parity with the same.