My impression is that while Qt has quite widespread adoption, KDE is still pretty niche. The majority of that I'd guess is that Qt makes a point to be cross-platform. Gtk since 3.0 is pretty well Linux-only and content to stay that way.

Gnome is still the default in most of the big distributions -- but arguably its most popular incarnation, Ubuntu's, deviates significantly from upstream's vision for the default Gnome desktop.

Some of these things are pretty hilarious to watch from the outside. For design/UX considerations, Gnome 3 hasn't supported desktop icons in a while now -- so a bunch of distributions ship it with one of the (several) desktop icon extensions, none of which are very functional, along with a bunch of other extensions, like Dash-to-Dock. So every time an unsuspecting user gets a work-issued Ubuntu laptop, you get to watch them trying to figure out how to drag a folder to the desktop, or pin a folder to the dock or whatever.

FWIW, though, KDE has a pretty wide install base in some popular distros, e.g. Arch: https://pkgstats.archlinux.de/fun .

It's an irony given that GTK 3.0 made a move for modularised rendering in hopes of getting more cross-platformable.

What does the Tk family lack to make "thrivable"? It doesn't have built-in resizable editable data grids, but that gap alone is not a show-stopper.

Qt is an application framework – it's much more than just a "GUI toolkit".

Tk is nice, but also small and very limited in scope, which is great for some use cases, but less so for others. It's like the difference between a desktop environment like GNOME or KDE, which do much more than just "manage windows", and a more basic window manager like i3 or the countless others that are out there.

Tk is a nice little GUI toolkit, but feature-wise it isn't even comparable to Qt in any way. Personally I started with Tk but it turned out to be too limited, so I switched to Qt.

GTK is actually used quite a bit more on mobile these days thanks to all the distros and software targeting Pine Phone and Librem.

A big hope it is a genuine resurgence, and not a GPE 2.0.

But without Android support, their chances are at least halved.

Unless Google doesn't allow play services to work in emulation, I don't why this would be a problem. Users can choose if they want Android compatibility or not.

GTK is thriving by all accounts. Could there be more devs on GTK itself? Yes. Are more and more applications being written in GTK? Yes. Look at Flathub and all the developers putting applications out there.

Which distros are shipping with KDE by default?

I know it's rhetorical, but... openSUSE

Slackware: Plasma 5 was just moved from testing this week in Current (for the upcoming Slackware 15).

Kubuntu, KDE Neon

SuSE.

SUSE Linux Enterprise Desktop currently ships GNOME (and IceWM) and doesn't even mention KDE:

https://www.suse.com/releasenotes/x86_64/SUSE-SLED/15/ https://documentation.suse.com/sled/15-SP2/

You may be thinking of openSUSE.

Xfce, Cinnamon, and MATE all have a significant user base and they use GTK.

For me personally, the takeway is that both GTK and QT are getting old.

React (and Svelte, etc.) have completely changed the way we build apps, and typical QT & GTK apps are still being written in a retained mode approach with signals & callbacks like 20 years ago.

QML is quite interesting but it doesn't feel immediate mode enough like React & Svelte.

Flutter, JetPack Compose, Crochet, Fidget, Revery, MakePad, Svelte, etc. all those bring so many new ideas. GTK or QT feel so complicated and hard maintain in comparaison

I don't know if that's true or not, but I just like Gnome so much more as a DE that it's a shame to read that.