The common suggested alternative payment system to fix the "unfairness" of Spotify's current splitting of the pie is "user-centric" payments: https://www.google.com/search?q=spotify+user-centric+payment

It's easier to illustrate that idea with a more extreme example:

  User 1 listens to nothing but songs from an obscure independent jazz artist and never any pop songs.

In Spotify's current payout system based on pro-rata streams count, most of User 1's subscription money will still go to popular artists on big record labels like Taylor Swift, Drake, Ed Sheeran, etc. The user-centric model wants 100% of User 1's money to go to that independent artist and 0% to Taylor Swift.

For the author's particular examples...

>Now let's take a different scenario.

  User 1 listens to 90 songs by Ariana Grande.
  User 2 listens to 10 songs by Billie Eilish.

>How should the money be fairly split? 50:50? 90:10? Something else?

Using "user-centric" model, 100% of User 1's payment would go to AG and 100% of User 2 goes to BE. From the perspective of the TOTAL SUM, this also means that 50% of the total pie goes to AG and the other 50% goes to BE.

>Imagine two users of Spotify. Both pay £10/month.

  User 1 only listens to 900 ABBA songs.
  User 2 only listens to 100 Beatles songs.

  90% ABBA, 10% Beatles ?
  50% each ?

Same math breakdown as AG and BE above.