Can't you just change the music in a silent update if the thing took off?

Sure you can. At 100k / month revenue, you have 100 currency / month or 1200 / year available for another artist, which may or may not include the risk of messing up the "feel of the SFX and music". Even at 10x that, that's a paltry sum, especially if identity is tied to it.

Honestly with it being such a small percentage you have to really hit it big to make such a dick move even worthwhile anyways