Well, a lot of the power of the UNIX shell comes form this and I see this as a major advantage over Windows. So no, I do not think Windows got it right.

Any kind of replacement should aim for the same conceptual simplicity and power. Sadly, I fear that people driving development nowadays are more interested in building unbreakable walled gardens for advertisement or app stores, or trying to squeeze down the some small gain when used on the cloud. I am more interested in general computing on the user side.

Having fd 4 mean something specific is no weirder than having fds 0,1, and 2 mean something specific, which is probably never going to change. At some point you just gotta embrace the Unix.

Is it weirder, that you can pass an variable precisely into argument 4? You do need to pass information to a subprocess and there needs to be some agreement on what means what. Sure, maybe you could use names instead of fds, but that sounds needlessly complicated.

Well, Cygwin and Busybox have shown me that fork-heavy activities are about 100x slower on Windows than Linux.

The Windows approach may be correct, but it suffers in performance from the POSIX perspective.

I have heard that WSL1 iimproves this.

You're simply failing to grasp the value of the simplicity, compatibility, and portability of POSIX/*nix. Inventing yet another way to create a process would be complex and break things. It's a-la-carte to enable configuration after fork of the new CoW or non-CoW process but before exec (unless vfork or similar were used). This is the model.

If you want to greenfield re-engineer the world with all new system calls and a totally different execution model, feel free to go right ahead.