[caf]

I think if you were building an OS an an academic research exercise and publishing papers based on it, no-one would object to your results on the basis of a lack of POSIX compatibility.

The criteria is different if you're writing an OS for practical use with wide adoption, though. There, backward-compatibility is rightly considered a positive attribute - there is much existing software in the world, and the entire point of a practical OS is after all to run the software that people use.

Exactly the same situation exists with CPUs. If you come up with a new micro-architecture that you actually want to sell to customers, you'd better come to the party with a C compiler and a ported OS or three.