Couldn't you replace "Google" and "Play Store" with "Spotify"?
You can "sell" your app directly to users through your own site (see Fortnite) or dubious sites like https://en.uptodown.com/android/general-android but you have the same "pull factor" with the Play Store that you do with Spotify (the user base).
The Google + Play Store and Spotify (company) + Spotify (platforms) situations don't seem fundamentally different to me, just different by degrees. Google's monopoly on Android app distribution is unparalleled.