"codebase already written in C" includes both "all the as yet unwrapped libraries" and "the OS interface".

There isn't. Fast code is pretty important though to a lot of people while security isn't (games, renderers, various solvers, simulations etc.).

It's great C is available for that. If you're ok with slow use Java or whatever.

No, and, in fact, the first one isn't valid - you can use C++ (or a subset of it) for the same performance profile with less footguns.

So really the only time to use C is when the codebase already has it and there is a policy to stick to it even for new code, or when targeting a platform that simply doesn't have a C++ toolchain for it, which is unfortunately not uncommon in embedded.