> over time for non-mainstream users because the some limited number of mainstream modes dominate as most statistically "optimal" across the total user pool

This isn’t true, YouTube recommendations when it chooses music are amazing (no idea if YouTube Music is good I mean the video site).

Spotify recs are intentionally recommending you things cheap to stream or that have been paid for. It’s not a raw rec engine and it’s not bad cos it’s collapsing under normies, YouTube is proof of that.