Based on the description, it seems like fair use. It's like transferring music from your own vinyl record to a tape. It's not up to other people to judge why someone prefers to listen to music from a reel-to-reel tape instead of a vinyl record. The same is true for streaming versus listening from WAV files.

I think if you transcoded all the music you want from Apple Music so that you could just listen with an MP3 player or PC of your choosing instead of their software, if you continued paying the subscription fee indefinitely - would be pretty defensible, ethically upstanding, and let's be real, there is a 0.000000000% chance you'd ever get sued for doing that on your own.

The hazards of course are that if someone were to do so and stop paying the subscription fee, they're in dubious moral territory, and if someone built a tool to "help" you do it automatically that person is going to be sued.