I agree with some of what you're saying; some of the well known risks of working in C are because it's a small standard. But much of the undefined behaviour was deliberately made that way to support the hardware of the time - it's hard to be cross-platform on different architectures as a low-level language.

C genuinely is easy to pick up. It is harder to master. And you're right, for many domains, there are better options now, so it may not be worth while mastering it.

Because it's an old language, what it lacks in built-in safety features, is provided by decades of very good surrounding tooling. You do of course need to learn that tooling, and choose to use it!

In the context of Magic Lantern, C is the natural fit. We are working with very tight memory limitations, due to the OS. We support single core 200Mhz targets (ARMv5, no out-of-order or other fancy tricks). We don't include C stdlib, a small test binary can be < 1kB. Normal builds are around 400kB (this includes a full GUI, debug capabilities, all strings and assets, etc).

Canon code is probably mostly C, some C++. We have to call their code directly (casting reverse engineered addresses to function pointers, basically). We don't know what safety guarantees their code makes, or what the API is. Most of our work is interacting with OS or hardware. So we wouldn't gain much by using a safe language for our half.