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.
The video streaming industry is moving to silos of content with varying levels of success which, regardless of the price, is bad for the user experience.
For example the HBO Go app is bad in terms of usability and (at least in my country) the streaming quality is really bad. I can watch 4K HDR in Netflix, no problem it starts instantly, but HBO Go always looks like some low bandwidth 720p content, specially on camera pans.
"DOS is still great, Why does it need a replacement?"
If something is robust and efficient, why does there need to be something new to replace it? It serves the purpose and solves the problem well, why would you replace it with something new simply for the same of something new?
DOS isn't a great example, DOS is a much bigger system than something like BT. BT is more of a protocol than a piece of software, and even in that case there are piece of software which have existed for decades without the need to be replaced for the reasons above.
New does not mean better, and just because something is old does not mean it's bad or there is something better out there. Perhaps we come up with something that comes after torrents, but we shouldn't do it for the sake of it, it should be because torrents don't work for a use case or we've come up with a faster / more robust way of doing it.
It works perfectly fine as it is in my opinion.
IPFS is probably spiritually closer to BT in terms of censorship and content sharing, and could be a valid replacement if the right incentive based sharing applications are built over it. Right now there is little incentive to seek out and add content to IPFS, if that can be cracked then BT could be wiped out by it.
However, the trend now for most people is to subscribe to spotify, netflix and amazon prime, thinking there is an endless selection of great and original content. Instead you quickly run out of that and end up listening to and watching whatever the algorithm tells you to.