Edit: I mean so the market does exist yes, but it seems to be filled by things like MP4 players (running Android) and old iPods with flash mods

Only because there isn’t a phone to offer the required functionality. (Unless you keep your last good option as long as possible like I’m doing, which has it’s own drawbacks). As I said before these features already existed and they removed them from new models for anti-consumer reasons. Not because there isn’t a market. It’s not like new phones got cheaper or had new features added when they took the other stuff out. They did it to sell more wireless headphones, make it harder to replace work batteries, and lock us all into their streaming and cloud services. The market isn’t served because they don’t want to serve it. They want the market to serve them.