Spotify serves mp3s because it uses less bandwidth and most people can’t tell the difference on their 30€ Bluetooth headset.

I think this highlights a bigger issue when it comes to this discussion.

The issue isn't the mp3 format -- for the most part, the format of any lossy encoder can sound good with the right settings. The problem is that, unlike flac, all encoded lossy files are essentially untrustworthy audio formats. So when people say mp3 sounds bad, it's only a half truth in the same way that it's a half truth to say that people cannot tell a difference. You are putting trust in the person who encoded the audio to make the right choice and the encoder is putting trust in the idea that the person consuming the media can't tell the difference.

When it comes to being cheap out on bandwidth since most users can't hear it, that's a huge cop-out being made for a company that can do better. While Apple is pretty notorious for making terrible decisions for arbitrary reasons, even they respect the user enough to allow you to opt into higher audio format quality. It's decisions like these that cement Apple as the kings of the creative computer user.