Siri, Cortana, and Google Now are mainly for collecting personal user data, and understanding user behaviour. Microsoft has already made cortana available on iOS and Android (though it is not baked into OS). Similarly Google has made their tech available on other platforms. Apple's data harvesting ambitions are not huge currently, mainly because of their absence from search and advertising business, but I wont be surprised if one day they make Siri available on other platforms too.