What do you mean by the "idea" of functional programming? Functional programming is not a "paradigm" or whim to be believed or disbelieved in - it IS what programming is. What are you going to program with if not functions? Functions and relations (morphisms) are what the nature has given us. That said, I feel that the comparison of functional programming with imperative programming is unwarranted. Imperative programming is just a particular way to implement a "function" where the programmer wants to manipulate state explicitly. One can very well program imperatively in a "functional programming language" like ML. So imperative programming is just a paradigm, or a style of programming. The important distinction (apart from the type system) is that 'functional' programming languages allow one to work with higher-order functions, whereas imperative languages like C do not facilitate it. One should nevertheless be able to write higher-order functions in C using a painful encoding, just as one would write down the description of a Turing machine on the input tape of a universal Turing machine.

In my opinion, C seems to be the result of a desire to get a high-level programming language mixed with low-level access to the memory, in order to be able to write an operating system. It just seems to be a bad design to mix different levels of abstraction in the same language. This mixing of abstraction levels actually makes matters worse for C - things start becoming OS dependent. Because of the low-level access to memory available, one can write C code that would do one thing on one architecture, and a completely different thing on another. This just blows away your argument of C being designed to be 'portable'.