This concern seems bizarre. Essentially every song is on Spotify (unless it's very small in which case it's on soundcloud or something). This isn't a unique property either - Apple Music replicates it and YouTube/vevo come close. There's good reason to believe that streaming services, or something equivalent/better will continue to exist and I will continue to be able to get the songs I want. When I listen to a song my primary concern isn't about being able to share it with my children's children or whatever. I'll probably forget about it by next year. How often do you dig up songs you listened to 20 years ago?
Hardly, but their selection is impressive. What music Spotify has available is entirely dependent on agreements with the artist or, far more often, the music label. Those agreements are subject to change or expire, and songs are routinely offered for a time then removed (sometimes to be added back later) and can be subject to additional limitations such as having their availability limited to specific countries. It's basically the same situation as Netflix with movies and TV shows. Sometimes you can find copies uploaded by others to youtube, pirate trackers, or other hosting sites and sometimes you can't.
Streaming services will likely continue to exist for a very long time. Streaming services that provide the specific songs you want to hear or enjoy on Spotify today are not guaranteed and that likelihood decreases significantly as time goes on and your tastes diverge from the mainstream.
If all you listen to are top 40 radio hits you'll probably never have a problem finding copies of the songs you like in some form or another. Whatever has replaced Spotify 50 years from now will likely still be full of The Beatles and Brittany Spears. Even today I find there are songs that are hard to find, albums that are out of print and not available from streaming sites. As people move away from physical media and depend on the shifting sands of streaming music providers I expect the problem will only grow with time.
> How often do you dig up songs you listened to 20 years ago?
Constantly. Not that I'm only listening to songs from 20+ years ago, but my current playlist has songs from the 60s, 70s, 80s, and 90s and most playlists I create tend to as well. Not everyone cares about music and may not mind if they never get to hear their favorite songs again by next year, but I suspect many of the younger ones who feel that way now will at one point in their lives want to go back and hear songs they grew up listening to.
Lucio Battisti is possibly the most popular songwriter in Italy ever. His songs were added to Spotify one month ago for the first time. And even now, the catalogue is not complete.
The Beatles today are there, but they were missing for a long time. King Crimson have been added in June.
I don't know what you mean by "essentially every song", but they are just now adding albums from the most popular artists, I would think many minor things are still missing
