The diagnosis for GNU/Linux is better than I expected but I think is still incomplete. Yes, you have two major toolkits (GTK+ and Qt) and many minor ones (most of which wrap one of the majors). Qt is proprietary but also available under a free software licence, but what if you don't want that that complexity? It feels like modern GTK+ is less of a cross-platform toolkit and more of a runtime layer for libaidwaita and the GNOME stack. So if you don't want to conform to GNOME's UI conventions, it's not clear where else to go.

Also, the explosion of new languages in recent years means having to write a new set of FFI wrappers around existing libraries, and it's easier to make an idiomatic library for TUI development than wrap all of GTK+ or Qt.

Qt's license really isn't hard to figure out. If it's free software, you use the free software license. If it's propietary/commercial, you use the propietary license.

The xfce folks are keeping GTK+ 3.x around, mostly for its advanced theming support. Isn't that quite enough?