it's an appealing project, and i think there's potential to do better than posix, but it's easier to do worse
Bingo. Personally, i always disliked POSIX and preferred NT's way of doing things. Synchronous IO by default is silly.
I do get your sentiment though. But it’s an unrealistic ask. Particularly for a hobby project.
But even if we take the MIT Lisp Machine system, which is smaller, it is still 450 KLOC. Just the microcode that implements the Lisp Machine on the CADR around 25 KLOC.
The Lisp Machine system is also _very_ complicated under the hood when it comes to the core system, and one of the reasons why Unix succeeded.
It is also a system that is a total PITA to modify in incompatible ways, since you are always modifying the running system. There is no such thing as an executable file in the same sense as on Unix that you can copy and keep around, e.g. if you modify the compiler on Unix you can easily keep a backup copy of 'cc' and keep running that when you screw up.
On the Lisp Machine you can't keep two compilers around at the same time without essentially creating a fork of it with all new names (and even then it might not work).
The good thing about POSIX is that you can theoretically port wine too at some point, and then you get both. (Though I am aware that this is not very easy either, would be interested in the opinion of somebody with wine porting experience.)
1. You want to run POSIX-compatible software in a context that existing kernels won't support.
2. You want to understand how POSIX works.
3. You want to write a kernel but not design it, and POSIX offers plenty of references.
4. You want to optimize something about POSIX implementations and needs a bottom-up approach.
If it had a custom interface, then porting large projects would be an enormous undertaking.
All the shenanigans with modern Windows aside, I am genuinely a fan of NT's design, think it does several things better than POSIX and Linux in particular (notably, the things you mentioned, plus anything to do with device drivers) and wish the open source OS world took more inspiration from it.
Yeah I know ReactOS exists, but it's held back by its lofty goal of being a complete, bug for bug compatible Windows clone with full Win32 userland, when I'd be happy with just an open source NT-like kernel.