The OpenBSD project was started in 1995, with ancestry going back further than that. Should they have first invented Rust? Or at what point do you suppose the decades-old codebase should have been completely rewritten?

It's not too late to start now, similar to how Linux did a few years back.

Rumours of Linux being rewritten in Rust are greatly exaggerated.

No one is claiming that, they are at least setting a foundation for that to be possible though.