Despite the advantages of functional programming, imperative and object oriented programming will never go away completely.
Imperative and object-oriented programming is a step-by-step description of the problem and its solution. As such, it can be easier to comprehend. Functional programming can be a bit obscure.
Ultimately, a useful program will always have side effects (like providing actual output to the user for consumption), so the purest of functional languages will still need a way to step into the imperative world from time to time.
The current state-of-the-art is that imperative languages (such as C#) borrow features from the functional world (such as lambda statements) and vice-versa.
