The two options are Apple integrate Spotify Connect, or Spotify integrate AirPlay 2.0. I don't have much faith in either option happening.
Bluetooth wouldn't be good enough.
Spotify does support Airplay. The thing is that still uses your phone as the source, and a whole lot more battery, while Spotify Connect hands over the playback to the target device itself. Most important in this case would be Siri integration, to this date it doesn't even let you open a song in spotify :\
Exactly, it's effectively a WiFi version of Bluetooth. Although I'm guessing notification sounds wouldn't be broadcast over AirPlay?
Just like Amazon, they'll probably learn that soon enough.