Phones come with like 128 GB of storage. Not all is usable but that accommodates like four days of continuous uncompressed music. How long do you really go without a network connection and need music for?
Also I think they playback with hardware don’t they? They aren’t running the CPU. It uses a trickle of charge just like your iPod.
IMO a person who wants to store music on their phone locally doesn't want to supplement streaming, they want to replace it entirely. If they didn't then you're right, all of the major DSP's offer local storage through their apps and the 128/256/512 GB options available from iPhones is fine. (no idea what the limit is like for Android phones, I'm sure it's much more)
But you already have a phone (I guess) so the cost you're competing against is zero.