>What's it with young people these days being overly excited over the idea of using ever unfinished window manager or building their Linux desktop from similarly ever unfinished separate small components.

Because it's fun, and it looks cool. It's all configurable through text, not unconfigurable, and most people that do Unixporn stuff like this seriously are usually beginner programmers who got acclimated to doing everything in the terminal anyways, so they don't mind, Vim/Emacs etc.
The cool thing about open-source is that u do what u fucken want.

>Use X11, use a proper, mature DE now, it's Xfce, KDE, or Gnome2 and help make it perfect.

I will only use the steaming pile of broken dogshite that X11 is over my dead body.
I love my KDE Plasma 6.8.1 & GNOME 51 though, pretty amazing desktop environments that recently released, I've been able to get them running on ancient toasters with Debian, Arch & Fedora and they're so fast, they even support HDR video editing!

I do also get your point, one should contribute to free software projects, atleast talk about it if you can't donate or program.
Actually, ricing is a very good way to do that, once you show people how cool your desktop lookz when u customize it, they'll be more receptive to Linux/BSD/plan9 etc.