Security is a factor everywhere. And C is everywhere. There's not going to be a "rewrite everything in Rust" project until after there is some legit penalty for selling / offering software in C that could be more secure in another language.

I think Mozilla got it right when they abandoned the effort to rewrite the entire browser in Rust and instead focused on analysis of which modules needed priority rewrites. If a module has historically had zero memory-related bugs, then there are other higher priority things to spend time on.

That said, Google Project Zero should have had Brotli on their radar. Hopefully they still have people and resources.

Because these applications were created long before Rust existed.

There is a cost to conversion.