Rust is—again for better or worse—something Linus thinks is good for the project, and thus learning Rust at least enough to not break the builds is a requirement for the project.
That misrepresents the situation. Linus accepted Rust provisionally, and only into certain parts of the kernel (drivers). It’s more of an experiment than what you wrote would suggest.
Rust is mainstream now,
Rust is highly visible now, due in no small part to its deafening evangelism. But it is not remotely mainstream in the sense of being a prevailing language, nor in the sense of being representative of the majority. It brings to the table a novel way to solve certain problems, and that is useful, but let’s not mistake that as the only way or those as the only problems.
Rust is mainstream now, and “i don’t want to learn this” is a dogshit technical justification.