As a WM user myself, it’s a big hassle to choose system utilities, and to manually write config or environment variables to have programs understand I’m using a custom DE and just behave like it’s GNOME, KDE or XFCE.

On the other hand, mainstream DE don’t natively support tiling. There are extensions or plugins do that, but there are a lot of problems with that. To name a few, 1) like said, they are sometimes bugged in edge cases; 2) I could report the bug, but it takes time to fix it, during which I have to disable the plugin; 3) when the extension devs abandon the project, I have to move on with a new one, which often behaves differently; 4) when the extension or the newest version of the extension requires newer dependencies, but I can’t install them because I don’t want to shake the whole dependency tree for my system

All aforementioned problems can be resolved with a DE that natively supports tiling, and as of now Cosmic is the first that does it in history, letting alone supporting Wayland as well. From that perspective, the project is not “just a rewrite of what’s existing already”

I’ve had issues where the tiled windows go all over the place before/after connecting to external monitors in GNOME Pop shell. I can’t speak for the entire Cosmic project, but as an end user who wants an established DE with native tiling windows that always work as intended, I consider the project justified