Ugh this was also what got me to stop my Apple Music and then Spotify subscriptions. Songs or entire albums can disappear simply because the licencing changed.
May I ask, why not both? I‘m pretty sure you will not buy 20mio songs, rather a couple thousand. With Match you get your own collection online and available everywhere, and augment it with the Music subscription. I’ve done it like this but only rarely need the Match fallback now, because most of my library is available on Music or Spotify.
Two reasons: cost and that I like things in one place and format.