QML?

Or if you want native applications, what is wrong with Cocoa, Qt, and XAML? A cross-platform toolkit will either be the lowest-common denominator between the platforms (this has been tried before, see e.g. AWT, SWT, wxWidgets) or something that is alien to each platform (Electron). Small independent development companies have done applications across multiple platforms by using their native toolkits for decades. And now suddenly, it is too expensive for GitHub, Slack, and others to develop applications with platform UI toolkits? They are just transferring the cost to the users. We get inconsistent shortcuts, widgets, etc. between programs. No automator support, etc.

There is only a benefit to people who use multiple platforms and want to applications to look the same on every platform. But the vast majority of users just uses one platform, or two if we include tablets/phones (which have different input methods anyway).