Spotify Premium costs £10 a month, so £120 a year. If I own zero music and use Spotify Premium for 10 years it will cost me £1,200. If I then decide that I don't want to pay £10 a month any more, I am back to owning zero music.
That is the potential downside of using Spotify Premium, as spending the equivalent £1,200 on CDs would mean that you owned around 170 albums worth of music that you could now listen to for the rest of your life for free, whereas with Spotify you have to carry on spending £120 a year for life to listen to your music, even if you are now listening to the same 170 albums over and over again.
Now, if you constantly listen to new music and are happy to spend £10 a month for the rest of your life then that isn't a downside and Spotify is the perfect solution for you. But if you spend less than £120 a year on physical music or iTunes downloads, or if you will eventually reach a point where you know what you like and are happy to listen to that selection, then it doesn't work out quite as well.
Don't get me wrong, I think Spotify Premium is an excellent service, but I do think that some people that talk about never having to buy music ever again are missing the big picture, and thinking very short to medium term.
I personally tend to subscribe to Premium for a couple of months a year and use that as a chance to listen to new music or artists that I want to get into, and then use the £100 that I've saved by not subscribing for the whole year to then buy the albums of those artists that I like.