How is functional programming not already successful? All new languages claim to be functional, old languages add functional features. FP is seen as the best way to use multicore and keep improving software.
I'm only pointing to Tiobe because it is popular, not because it is accurate, but you could use any other popular index and you would get similar results: a whole lot of imperative languages, but none of the officially Functional languages.
How is it not successful already?