While I agree it’s better to redesign apps that need help — I also think it’s a very good learning exercise to redesign something you use daily, because you have a lot more insight on what you would change.
I don’t use Spotify, so safe there, but every time we do our little “design hackathons” (weekly, around client projects — to stay in shape) we choose an app we use.
We were lucky/unlucky enough to so far only redesign horrible experiences and I can sadly say the best designed app on my phone right now is Uber and it’s not even THAT great (it is really good, but not groundbraking).
Especially apps in Poland are done in a way that I call “software house design” in which a software house asks for 1–2 screens and “the developers will code the rest”.
Usually with poor alignment and complete chaos ;)
So generally I agree that there should be more “redesigns” than the same app over and over, but at the same time when someone’s learning let’em at it! :-)