It’s not the plan for me, but I think it’s not a terrible idea. 1) Apple probably has usage data that shows that a large number of Apple Music users exclusively use it through Siri. 2) It’s more price competitive with other services now. 3) It could sell Apple Music to the Spotify crowd. That is, while I happen to like that Apple Music gives me a proper library and integrates with my existing library, there’s a whole generation of young people who probably have no conception of physical media, albums, or playlists and whose only exposure to music has come from the radio and services like Spotify.
And, honestly, while Siri isn’t great at individual songs (I suspect no AI assistant is due to natural language processing being hard, especially with things as open ended as song titles, but I only have experience with Siri), it’s pretty good at things like “shuffle my library”, “play some [adjective] music for me”, that sort of thing. And it’s probably better at pulling up more popular artists (“play some Ke$ha”) than more obscure artists (“play some Schooner Fare”). And I’d imagine that this is how most Apple Music users actually use the service, even if I tend to use it more as “iTunes in the Cloud+’free’ tracks from the iTunes Music Store” or as a cloud based jukebox in free play mode.