The total pie has absolutely grown. There were a little under 1 billion US album sales in 2000 (the peak of CDs). Spotify alone paid around $3.5B in royalties in the US last year (with similar #s for apple music and a bit less for YT music).
I suspect the disconnect comes from
a) a big increase in the # of artists
b) artists trying to compare apples to oranges #s as though every stream would've been an album/MP3 sale
c) the timeline of revenues: an album sale is a big cash flow shortly after the album release, but streaming revenue is a slow trickle as users gradually discover the album, listen, re-listen, etc
I suspect the disconnect comes from a) a big increase in the # of artists b) artists trying to compare apples to oranges #s as though every stream would've been an album/MP3 sale c) the timeline of revenues: an album sale is a big cash flow shortly after the album release, but streaming revenue is a slow trickle as users gradually discover the album, listen, re-listen, etc