Ritchie and Thompson also sort of wrote C, at least, some parts of it. I fully understand when Mozilla writes Rust and Servo using Rust.

No need to limit things to POSIX compatibility, but merely achieving it is no mean feat — and POSIX is seriously C-oriented. Some people invent a new OS architecture, then implement POSIX on the top of it (see NT) — but this is an order of magnitude harder, I suppose.

When you implement a POSIX-compliant OS, you suddenly have a lot of ready-made open-source software that both serves its primary purpose and as an "integration test suite". For a new OS, you have to write most basic userland things yourself. This is again a lot of effort.

Order of magnitude strikes me as too high if you think about doing it ahead of time and factor it into your design. The harder part would probably be not letting POSIX compatibility cramp your design. For example, the use of fork() is in part an artifact of making a practical system run on very memory limited hardware (like 128KiB max for a program, half code, half data), there's no intrinsic need for it to develop solid software.

Some very smart and pretty motivated folks at GNU/Hurd project struggled for years with a similar, but a bit less ambitious, approach. You never know how deep a puddle is.

> Ritchie and Thompson also sort of wrote C, at least, some parts of it.

Given its open nature, there's no reason (except limits of time/skill) someone writing an OS in Rust couldn't also be advancing Rust and its libraries in the directions needed to support that effort, even if they are outside of Mozilla.

> except limits of time/skill

I won't mind to be a billionaire with superhuman abilities and unlimited lifespan, too! Unfortunately, we mere mortals often have to limit the scope of our endeavors somehow.