…sure. Yes. If you own a song, you can listen however many times you want, simultaneously or not.
But streaming services are simply a different value proposition. Listening to an mp3 means either buying all the music you listen to or pirating; it also means having the music stored on your listening device in advance, or streaming from a personal media server. I listen to a lot of music that I haven’t heard before and don’t know if I’ll actually like; I also listen on my phone a fair amount and have a limited amount of storage space for music. For that use-case, streaming is preferable (to me).
I mean…okay? My whole point is that “purchase a bunch of music to own forever” and “pay a streaming service to hear a bunch of music once” are totally different use-cases. It’s great that you own music. Good for you. I own some too! But streaming fits my needs better overall.
And all of this is completely beside the point that it’s really not that weird that the cost for two people to stream is higher than the cost for one person to stream.