They've already made a few attempts over the years (Windows Subsystem for UNIX comes to mind), neither really caught on, except WSL.

I also don't quite get why one would want such a setup - why not just use MSYS2 or WSL? As it is, it's just a mishmash of CMD builtins, Windows utils, Powershell, and these Coreutils. Will one have to use CMD-style (%var%) variables or will it be the POSIX way ($var)? Also just keeping in mind when to use /s or -s style switches, which version gets invoked depending on the PATH, PS aliases, etc. is just a lot of mental overhead for seemingly little advantage over WSL.