I'm less familiar with the Microsoft ecosystem, but I'm pretty sure it is; Microsoft named their libc MSVCRT.DLL, but it's still the canonical system ABI, and NT, AIUI, actively shuffles system calls between builds to prevent people even trying to use them directly.
The rest, and your general point, are fair, though; there's nothing preventing the creation of a system that doesn't work like this.
No, msvcrt.dll isn't the system ABI. It's officially not even part of the OS and you aren't supposed to rely on it existing. Kernel32.dll is the primary entry point for OS things, and it's distinctly not a libc.
I'm less familiar with the Microsoft ecosystem, but I'm pretty sure it is; Microsoft named their libc MSVCRT.DLL, but it's still the canonical system ABI, and NT, AIUI, actively shuffles system calls between builds to prevent people even trying to use them directly.
The rest, and your general point, are fair, though; there's nothing preventing the creation of a system that doesn't work like this.