Codes for music subsystem doesn’t sound like something that’d be updated/maintained often, so it’s possible it could be very old. (Worst case it could be carried over from 1.0 and just padded with workarounds every time a fix needs to be made)

Adding the ability to play user-selected playlist in the open world + cities would be a sweeping change to the existing music subsystem. Even if the coding itself end up being simple to do (which may or may not be the case), they’d still have to test it for every place in game where they use music. In other words, every field, city, duty, and in-game cinematics.

Obviously no one outside of FFXIV development team would know how easy/difficult it’d be to rewrite/refactor their music subsystem. But at a time when they are expected to be focused on developing new contents, changes to something that is already considered working, like the music system, is likely not going to get priority.