We have hope on the horizon. The Ladybird browser won't look so out of place on classic UIs when it reaches maturity.

Also Windowmaker. I have been using it for years back in the day, and just to be sure reinstalled it a few minutes ago to check how it behaves today: no problems whatsoever with Firefox, LibreOffice, Gimp, Handbrake, and other modern apps. Using it right now to write this post with Firefox, in fact. Not that I didn't expect it to work since the last stable version is dated 2020 and the GitHub repo is quite active (last update 3 days ago).

Nostalgia aside (I was lucky enough to attend a real NeXT demonstration in 1994/1995) WindowMaker has been literally a life saver over 20 years ago when I had to build a desktop for ~50 remote points of sale in which the operator could only perform a few tasks, and setting up it as a locked "kiosk on steroids" with a fixed dock that could open only the 4-5 needed apps allowed them to do their jobs without distractions or exposing the unnecessary/dangerous parts of the system.

https://www.windowmaker.org/

https://github.com/window-maker/wmaker

Windowmaker is my go-to on systems where there is no hardware acceleration so they get bogged down with anything trying to do compositing.

Not saying it doesn't work. Just saying that the whole aesthetic is very easily ruined by a few apps.

I used WM actively in the late 90s as my primary. I still put it on any Pi or things like that that I need a quick desktop for. So clean.

Yeah I saw a NeXT demo in same time frame at Atl Comdex. That display postscript was coolio.

A buddy of mine had a NeXTstation in '90 (or '91, memory fades). Sure it was nice, I mean really nice. The reason I wasn't completely blown off my socks was only that we observed the Amiga in '85 (a common friend even had one). For us youngsters those five years seemed like a long time and the progress was expected. DOS/Windows seemed like a regression to us. I'm sure these days the juniors wonder why we geezers bother with Linux and not use Android instead ...

I've been using Window Maker since the early 2000s whenever i use Linux. I have tried other window managers (including a long-ish stint with GNOME 2 back in late 2000s) but always end up going back to Window Maker.

Also occasionally i hack on a GUI toolkit in C that i use for a few utilities (mainly a music player and a filesystem browser) and i hope at some point to make it good enough to use as a backend for LCL/Lazarus. Here is an old shot with various example programs/tests and the two utilities i mentioned:

https://i.imgur.com/cMxfxQg.png

I really liked CDE on my Solaris x86 box when I was in college. I'd love to see a "modernized" version of it. Modern renderers w/acceleration, slightly updated UI but keeping with the general feel of CDE, etc. I love macOS, but there's something about the old Irix or CDE environments that made you feel like you were "working" and not just using the same old computer as everyone else.

Xfce + a GTK theme that mimics CDE is probably a good option: https://github.com/B00merang-Project/Solaris-9 I couldn't get this theme to work on a modern gnome/GTK4 environment, sadly, but suspect it would still work with something GTK3 like xfce.

I think you touched the real problem here, there are many telented people willing to work on great GTK themes, but it's really hard to maintain themes when gnome is changing the API all the time... So most people give up.