...and a desktop environment isn't? Building your own GUI toolkit isn't that bad esp. if you have a team. Cross-platform isn't that hard, windowing is like the only thing that comes to mind where there are some differences in behavior.
Rendering would be done presumably using some cross-platform GPU API like Vulkan or wgpu.
> Cross-platform isn't that hard, windowing is like the only thing that comes to mind where there are some differences in behavior.
And let's not forget the accessibility systems, clipboard handling, filesystem (e.g. a GUI toolkit needs file open/save dialogs, which means you want to monitor the filesystem for changes, which is different for all operating systems), audio, notification systems, printing, ... And probably many other things I can't think of right now which modern toolkits like Qt or GTK have to deal with.
A full GUI toolkit is already an incredibly ambitious endeavor and cross platform compatibility only makes it much more unrealistic to achieve in a timely manner.
A desktop environment on the other hand can be much less work and can grow more naturally. For example you can start with a window manager/compositor (which there are good libraries for), launcher and dock/panel. All the applications (file manager, image viewer, ...) can be leveraged from other DEs and replaced step by step if needed.
I agree, I think building GUI toolkit first is a better idea than building a new DE before you have a better foundation. Not sure what's their rush with DE then. They can improve KDE instead, until Rust based GUI toolkit is ready.
Cross platform here is not easy, if you want that toolkit not to be ugly and have some kind of native look and feel.
They could limit the scope though. I.e. focus on Linux only from the start at least. But it's probably still a lot of work.