The truly unfortunate thing, for my listening habits at least, is that finding what I want on Spotify is always a crap shoot. They may have it and they may not. What I end up doing at the moment is paying $10 / mo for Spotify premium (primarily because I hate their ads and genuinely prefer the higher audio quality on my home stereo system). and end up supplementing with another $10 / mo in iTunes bills to buy the songs that aren't available on Spotify.
While I do enjoy the "instant gratification" of Spotify and the ability to get an "all-in-one" package, people's expectations and habits around music consumption are fundamentally different than other forms of media in that music is primarily a passive experience with reading or watching television / film content are both very active experiences. I don't think a precipitous drop-off is coming to the downloading world until those habits change--or until Spotify and it's ilk can cater to them (e.g. full library available offline easily, larger catalogue coverage, etc.)