I mean, people have lost plenty of stuff with iTunes upgrades. I know people who lost rare versions of songs when they uploaded them to Apple Music because it deleted the local version. And I lose a ton of metadata most times iTunes is updated. I definitely get wary of a software update from Apple, at least on Windows. Which is why I'm thankful they aren't touching it yet!
I have no idea how this will work for me as I've never used Apple Music and just have everything in iTunes. Will I have to subscribe to Apple Music to listen to the MP3s I have?
You shouldn’t. I imagine you’ll just have to go to “finder” on a Mac to add new music instead of iTunes