Assuming you meant to write, "Spotify could have been as streaming service _and_ an MP3 shop" ...
No, Spotify didn't have the leverage in negotiations with Big Record Labels to offer streaming plus DRM-free mp3 files for 0.5€.
Apple is more powerful than Spotify and they eventually relented to Big Labels demands and added a more expensive tier of songs at $1.29 instead of all songs being just $0.99 pricing.
Too generalize... to consider the realism of any hypothetical alternative business models for Spotify to have, you also have to consider whether the Big Labels would have even made those ideas possible when Spotify was negotiating the licenses.
It has never been easier to own an easily copiable and backup-able form of the music you want and use it with myriad ways to play it and listen to it.
People just don’t want to pay a dollar or two for their favorite songs.
Interestingly, Spotify you have been as streaming service and an MP3 shop where you actually pay the music like 0.5€ so you could actually do whatever you want and listen to your music the way you want.
The current situation is that you don't own anything and you can't just deal with raw files (which locks you into a quite shitty UI).