The stock answer is that neither will or should replace the other - they are different tools that have different sets of pros and cons, and which approach has the edge will differ depending on the project and other "soft" issues like the available talent pool.
I think you're right that the growth of concurrency due to multi-core will increase the percentage (of the global set of development projects) when functional programming is chosen over other styles.
I think it's rare today because the majority of today's professional talent pool is most comfortable with imperative and object-oriented technologies. For example, I have more than once chosen Java as the language for a commercial project because it was good enough, non-controversial, and I know that I will never run out of people that can program (well enough) in it.
