On the contrary, C is excellently designed for its intended purpose: as a language for writing UNIX. If that's what you're doing, C is the ideal choice. If you're using it for something else, you're misusing it.

What do you use to optimize a bottleneck in Ruby or Python? (After you have profiled, exhausted the higher level optimizations, and all that.)

Well, most languages use C for their FFI so you're almost stuck with that. You can still use D, since it can call C functions transparently. Though, if I were designing a high-level language and wanted it to be able to call a fast lower-level one, I might choose Forth or OCaml. Once it matures a bit, I might change my answer to Fortress.

If you take a serious look at the constraints under which C was developed, the systems it was to be used on, and the problems it was intended as a solution to, you will see that it is most excellently designed.

Unfortunately for C, the constraints have changed, systems are orders of magnitude more powerful, and people have attempted to use C for many things it was not well-suited to.