Not supporting Spotify might sell some number of new $10/month Apple Music subscriptions, but supporting Spotify properly would sell some number of $349 HomePods that wouldn’t otherwise be purchased. Apple makes money from both hardware and services, and my gut says they’ll make the most money by making HomePod more useful, even if that means opening up an SDK that allows for competitors to Apple Music.
I think it would behoove Apple to add a Siri domain for audio playback. Summoning Overcast or Spotify via Siri would be excellent and make the device more attractive. I currently have both Spotify and Apple Music as they serve different purposes for me, but that isn’t something most people should have to do.