The fact that people care a lot about Unix clones is significant, though. nine_k could have been more effective in arguing the point, but it seems like a strong point to argue. Do you think you Go is flexible enough to write a Unix clone with performance equivalent to a C-unix? If so, why has it not been done?
There is Biscuit from OSDI 2018 https://pdos.csail.mit.edu/projects/biscuit.html
Besides the sibling Biscuit, maybe because no one bothered to do it?
As simple as that, not everyone of us is a Linus.
I should also point out that if you are using a Mac, changes are its iBoot Safe C might already been replaced by Embedded Swift, a GC enabled systems language.
Chapter 5 of The Garbage Collection Handbook, or A Unified Theory of Garbage Collection paper for the incoming replies related to RC.
People care about UNIX clones because they are lazy, UNIX has the source code available, and an existing ecosystem that they don't want to replicate, so it always ends up being yet another clone, thus throwing away all the possible innovations.
We see this happening even with Haiku, Genode, Redox OS, or Windows now shipping alongside Linux on top of Hyper-V.
Unless one is an Apple or Google, with the money and will power to push something out the door, using Objective-C, Swift, Java, Kotlin, with plain C and C++ standard libraries, and even then people will bend backwards to put UNIX into those systems, even when the platform owners went to great effort to hide it under the official userspace APIs.