I had a while ago calculated the actual download cost with S3's pricing on what each ~100MB Spotify update would cost if it was self hosted. Assuming each paid and free Spotify customer on the iOS platform updated the app was something to the tune of tens of thousands of dollars depending on how you estimate the marketshare of iOS for Spotify versus their other platforms.
What the App Store provides is not valueless, quite the opposite in fact. It's an entire system of distribution management.
Is it monopolistic? By definition, no, because Android is the #1 smartphone platform in the world.
Is it great that Apple has provided no alternative way of downloading software to the iPhone and iPad? No, that's not great either, but users knew that from day one (when the iPhone had no App Store at all).
In reality, iOS is no different than a game console like the PlayStation 4 or Xbox One, and yet we consider those closed marketplaces completely functional and normal.
If Apple was the only company making smartphones or if they even made a plurality of them, perhaps this would be a problem.