For a DE to succeed it just has to succeed over lots of alternatives in some features.

GNOME

is widely developed and protected, as it is the default on Fedora (with Redbat) and Ubuntu (Canonical). It is kinda fancy but its main focus seems to be a new "material-ish" simplified and streamlined Desktop.

Just not with transparency, thin animations, blur, 3D Backgrounds, an actually working, goodlooking and existing panel/dock, Apps that actually use your huge top bar used for decorations. Compared to macOS or something.

So you could say its design philosophy is inspired by macOS, but with many different ideas like that "Virtual Desktops or die". But its basically macOS but less fancy, with a material-ish design like Android 13. I hate both hahaha.

KDE

Then there is KDE, which is really at the edge to look like the extremely outdated look of Windows 10 with these ugly rectangles everywhere, this no-round-corners fetish back in the day, "rectangles are elegant and good UI". Such a completely weird step back in Style from Windows7, with less colors everywhere.

I have to say though, that Windows11, apart from a lot of stuff like their new design framework for Apps that wastes space, while also in part just already look really fancy, looks way better than KDE 5.

So KDE is now in the position to develop own ideas, but they pretty much go in the same direction as in windows, their new panel changer applet by Niccolo is veery similar to that, and it looks awesome! So I am certain the KDE6 will improve in Design a lot, even though I think I havent yet tried a KDE6 Plasma where projects like Dolphin where already ported to KDE6.

Projects like Dolphin are just great. I found it so strange and new back then to use a file manager with a name XD but funny, that Dolphin, Nemo and Thunar all come from the sea. Dolphin, Ocular, Spectacle, Ark, Kfind, KDE-Partitionmanager, Gwenview (something like Gimp but just for light editing, while still being in an unsafe language with security holes everywhere). These are all just great and unique software projects.

XFCE, MATE, Budgie, Cinnamon

Afaik these are all using GTK, so you could see them as outdated GNOME forks. Maybe thats very mean though, I see that Budgie will soon have Wayland, so I consider it an actively maintained Desktop.

I honestly can't say much of the other environments, although I guess XFCE is based, had an okayish Design language that is at a solid base between Windows7 and some old Android.

Maybe if you tweak it it gets really modern, but I have looked enough Linux Scoop to know that every DE can look fancy with the amazing Community Designs. Actually Desktops should let random nontechnical Designers that dont understand Git make their Design. Like, no Code touching at all, just images please.

LXDE, LXQt, Window managers

Okay so these are basically energy saving DEs. They dont support lots of stuff simply, with their Design being at Windows XP or earlier. If you dont even have a menubar, that doesnt have to be styled.

So the Desktops maybe for old hardware and energy saving. If they have Wayland, so they can easily be used as the Window Manager of a Desktop, some Distro, Budgie or so, is looking to use an existent Windowmanager as their own. Just not with their own big Design Language, so they can make it "the Budgie Window Manager" without much problems.

I tried Fedora Sway and that was basically broken? My mouse was deduplicating and spamming the screen full with mouse symbols, meanwhile I could look at that... modern and colorful Titlebar, no Viridis or whatever actually thought through Color pallet of child crayon colors on a rectangle Bar with Terminal Font? Who wouldnt love this much "basedness"? But it broke, so yeah uninstalled that.

Cosmic, Hyprland

Now here come new projects, that actually have Designers working on them. Aaand in the Case of Cosmic it is entirely written in a damn safe language.

Okay its fair to say that Hyprland is a cool looking Window Manager, but Cosmic is doing something insane here.

Nearly all these bullshit old buggy File managers are in some C/C++ code, that just always breaks.

These are often 15 years old projects, they said "they cleaned up the packages" just by porting them to Qt6! Imagine what background changes they just left, because they would actually need to cleanup everything?

So Rust. Slint is said to be Qt-like, but not relying on C++, making it an incredible pain to write in Rust as you need to do some OOP translation that I dont understand because I am not in IT.

So Cosmic is doing something crazy and kinda new. Their interface was starting basically as a GNOME extension, quite literally. And there are many people layering just that package on their Fedora Silverblue (or future secureblue?)

But it is still basically imitating GNOME, with GTK support being one of the first GUI Framework integrations to come. They are doing nice material android-y things with their apps, and I imagine they are going in some direction here. Much like Android actually.

So COSMIC is a great beginning, but I think Rewriting something similar to Qt, with loots of frameworks and GUI Interface creation tools would be a statement. Like the Qt Creator, its crazy, a bit like this stupid TinkerCad but actually working and not being Windows-only and blocking Adblockers without even displaying Ads.

It would probably be "Qt stuff + Rust + Translation", as the Qt people will likely not switch to Rust just like that. But having the core Apps, Kwin, the panel rewritten in Rust, that would be incredible.

Kwin is already working kinda well, but now there is the Cosmic Desktop with whatever WM and they could just reuse that and adapt it a bit.

So yeah, its a big deal for Interface Frameworks to rewrite all their stuff in another, extremely different language.