I've used Linux for a very long time. "Excessive framework fragmentation" hardly describes the current GTK & Qt/KDE almost-monoculture. In fact, I can't remember the last time when Linux (or any Unix derivative) was so quiet in this regard.
Desktop Linux was anything but stagnant ten years ago, when there were at least three frameworks in current use (Gnome & co., KDE & co., wxWindows -- although that's not a full application framework) plus a lot of other UI frameworks that still weren't dead (Motif, Jesus Christ...) or we had not yet given up on (GNUStep, which is an entire application framework). And there were the people writing GTK software not intended to necessarily run under Gnome (which is being increasingly discouraged by the Gnome team now), as well as people writing Qt software without KDE's framework. Ooh, and Tcl/Tk wasn't dead and buried yet.
If anything, GUI software on Linux appears stagnant because we're in one of those CADT rewriting phases that jwz was writing about twelve years ago ( https://www.jwz.org/doc/cadt.html ). There hasn't been much improvement in the last three years, indeed; that's largely because Gnome 3 is struggling to come on-par with Gnome 2 in terms of features and flexibility, and as KDE finally emerged from 4.x's perpetual beta, it got into 5.x's. And, between KDE's increasing lack of relevance after everyone ran away from early 4.x releases, and Gnome's agenda for world domination, there's hardly any room left for developers who have neither a passion for architectural astronautics, nor a taste for political partisanship, so very little desktop software gets written outside these two.