I haven’t updated past iOS 12 so maybe things are different now, but at least there—while you can technically uninstall music, iOS will routinely ask if I want to restore it when e.g. someone airdrops me an mp3 file. And of course, I can’t change the default app.
If your device supports it, I strongly encourage you to update. Aside from not getting security updates anymore, which places you at significant risk: the privacy settings in iOS 15 are very good. Even location privacy for system level stuff can be highly locked down.
I have an 8, and iOS 15 doesn't feel "slow", if that's your concern.
Airdropping an mp3 results in a “open with” menu from which you can pick any suitable app. In fact, Music isn’t even in the list of apps presented. The Files app can play mp3s natively in a QuickLook style, no Music required.
