Linux

You say it's "needless" complexity. But that's what's up for debate, and most people, including Linus seem to disagree with you.

It's not a matter of whether Rust is demonstrably superior and more secure, that it is seems to be the common understanding and agreement.

A new project matching reasonable Kernel feature-parity would be too much effort. It's unrealistic.

The value is in moving the Kernel itself into a safer space and tool-space.

The idea that a technically superior solution would naturally supplant an earlier one with a huge market penetration and stability is wishful thinking. We see it in many areas. Without significant issues people at large will stay with what they know and what is popular.