glibc -> wayland -> gtk

I started a war answering there. You want a monolithic standard api? Great you do it.

You know what you'll get? Even more options.

Win32 was an early widget set for Windows, Microsoft created a new one every few years until 2012, when it came out with the unwanted Metro. Since then it's been unclear what widget set Microsoft wants users to use, "use Electron" seems to be as official any answer.

In the case of Linux there are an even larger number of widget sets: see

https://en.wikipedia.org/wiki/List_of_widget_toolkits

There is none

Motif / Lesstif / CDE, I guess?

If I understand what you're looking for, GTK. If it's a linux desktop, there's a super high chance that GTK is both installed and familiar to the user.

However...

> This would make vendors write more gui software for Linux

What makes you think vendors want the native look and feel?

Even on Windows, there's a long history of apps wanting to feel "unique". That's what they want: full customization to fit their branding.

Ncurses?