In case you’re not aware, there’s a setting to show unavailable tracks greyed out in your playlist, so at least you can tell which ones are missing.

Best I can tell, this feature is only applicable if the track is not available in the region you're in, and/or for tracks that are 'local music' that haven't been syncned to the device you're playing on.

If a song is removed from Spotify entirely, it just disappears from the playlist.

I'd love to be proven wrong, however, and start to see all the tracks that have disappeared in the past!

Where is that setting on iOS?

Yeah, it's already happening, that's why I brought it up. Right now it's not a huge problem, because it doesn't happen too often or on a large scale. Like you said, it's usually a couple of songs in a larger playlist.

Now, imagine the scenario comparable to what @bogomipz said above: "no Beatles, Pink Floyd, No Motown, No AC/DC". I'm betting all of us would be a lot more frustrated and irritated than we are right now. Maybe you won't do anything about it. Maybe I won't, either. But it sounds plausible that a lot of people will, whether they switch to a different service, come up with a solution of their own or simply revert to piracy.

I’m curious. Do a lot of people actually use a single music streaming platform? As an avid music lover, I get my new doses from various services. Spotify is the only one that I pay for. I mainly use it to discover the latest mainstream stuff from genres I don’t typically listen to. But the Beatles? Come on, why isn’t that saved in your hard drive somewhere?

Because it’s included in my Spotify subscription. Why would I pay ~200 dollars to “own” all the Beatles albums when music ownership is basically worthless?

Why would I save music on my hard drive when I have music streaming services?

I don't think most people are avid music lovers, and many people probably don't even use a computer for music at all, just their smartphone.

I don't think they would switch to another service, because another service is likely to suffer from the same problems of rotating availability. The only service I know of that doesn't suffer from that is piracy, which I don't think people will go back to on any significant scale.