C will be relevant till the heat death of the universe. if humanity ever dooms itself back to the stone age, all it would require is some bloke to invent a rudimentary binary computer and some nerd to write a basic C compiler for it, humanity will doom itself again in less than 50 years.

Unfortunately, it's the only sane cross-language ABI option there is.

C++ is a close second, which is mostly because C++ uses the C ABI wherever it can.

Even if the language itself is obsolete, it will live on for many more years just because of that.