This is not true at all. Songs do come and go. Labels have the right to pull their content down and this has happened many times before.
Also, songs disappear due to regional restrictions as well.
this, this, this.
tl;dr: spotify is awesome in concept but kind of evil in practice.
spotify is plagued with issues people here don't seem to be acknowledging, not the least of which is that there are giant gaps in its music coverage--and I'm talking about popular music, here, not just obscure records for music nerds like myself. the music industry is rapidly adopting a model in which an artist's best-selling archive records are
not available on streaming services, but their lesser archive records and any new releases are. also, some
very popular artists are not available on spotify--do you like the beatles, zeppelin, metallica, or AC/DC? not on spotify.
the spotify mobile app is possibly the worst of any professionally-released app for iOS I've ever used. it's exceptionally buggy and requires me to do things like force quit it over and over or actually restart my iPhone to get it to work properly. it's counterintuitive and difficult to use, to say nothing of actually conforming to apple's UI guidelines.
then there's the ethical issue of spotify: their payments to artists for streams are laughably low, so much so that lady gaga famously made $162 last year in one month for over 1 million plays. that's not a typo.
but do I have a subscription? you bet. I listen to so much new music that I would go broke if I bought it all, and this saves much more time than downloading it from soulseek. they have a corner on the market (don't even get me started on rdio or any of their supposed "competitors") and until people stand up for improving the service, they will continue to be another facebook and turn a blind eye to their consumers.