Spotify actually does have a very large number of songs, and I have no trouble finding classics in many genres - as well as completely new work.
That may be true, but my anecdotal experience shows that I will more often have specific current artists or songs suggested to me than any classics or long-forgotten gems, lending credence to the theory that Spotify cares less about what I'd like and more about what it wants me to listen to
For me, what often replaces Bittorrent for music is YouTube; still built on copyright infringement, but very unlikely to be blocked or get you in trouble, and it's easier than Spotify (no need for accounts).
e.g. When you pull up an auto-generated playlist or look at recommended suggestions, are you just looking at content which the provider prefers to show you? Can you trust reviews from the same company that's selling you the content? If some content isn't on there, will it just disappear into the abyss? If the provider decides to drop some content, will you ever see it again?
And yes, if content isn't on Spotify for me, it basically doesn't exist. If it's on Bandcamp maybe I will download it.
Spotify allows you to see songs that you have added that have been removed from availability as greyed-out names.
Large is relative. They have a larger number of Songs, but the actual choice is quite small. It's limited by age and countrys, and even types. They Catalog consists mostly of popular western music and music from the last 10-15 years it seems. Non-Western music or less popular music from the 20th century is quite hard to find on spotify, or any other music-streaming-service. Youtube is a better source for this and they have limitations too. Spotify used to have also audiobooks, but I think they disappeared in the meanwhile because of competing commercial audiobook-streamingservices.
We live in a great world today with all the information and entertainment cheaply available for everyone...as long as you just follow the local mainstream and have not much demand on specific details. It's great and sad at the same time.