Amazon might be close to launching a music streaming service according to the New York Times. According to them it could lunch as soon as this week but with a limited number of songs. It would be part of your Amazon Prime membership.
Now for me I don't think this will replace Spotify for me, but I can see how maybe one day it just might. Going by the NYT article they don't have a lot of deals with the labs but I'm sure over time they will get the deals they want and then services like Spotify will be in trouble.
Right now Spotify has the advantage of a free tire even though you can not play song individually. This free tire will probably keep Spotify around even if Amazons streaming service gets the deals they need to have as many songs as Spotify or their competitors. Spotify can run you $9.99 a month for premium though after a year you are paying about $119.88 for premium while Amazon Prime cost $99 a Year. Though keep in mind if the new York Times article is right you wont have the selection like you do with a service like Spotify.
We will see if this happens when the service dose launch. I will take a look at it when it comes out.