It's my view that music discoverability should, in 2018 and beyond, be a central consideration in choosing a streaming service, and it should be a central element in any review on the subject.
Couldn't agree with you more!
Up until Spotify came out with a subscription service, which now seems likes years ago, I carefully managed all of my music collection in iTunes. Until one day I asked my coworker why he was so fond of Spotify. His reply was simple: "It is so easy to discover new music".
In the old school music economy, where you had to purchase songs one-by-one, there is a bias towards listening over and over to the songs you already have. After all, you've invested so much money in your collection, it makes sense to maximize your return, instead of spending more. 30 second previews of the songs left "music discovery" feeling like work instead of joy.
That all changed with Spotify and YouTube Music for me. I now enjoy discovering music with algorithms doing the hard work and I love the fact that having a subscription encourages me to explore rather than re-listen. Occasionally I refine my music preferences by giving algorithms signals ("liking", "thumbs up" etc.) as I listen to the endless stream of music, which generate better mixes and recommendations.
I found that YouTube Music actually has better recommendation engine and serendipity than Spotify. But I use both for different contexts: YouTube Music for Apple TV music videos, while Spotify for songs. I still create playlists though because there are songs I just can't get enough of and having a quick access sometimes is mission critical.