The software running on a specific ROM might not be able to change (assuming we're talking for devices using mask ROM and not EEPROM) but it doesn't mean the code itself is supposed to disposable. Different devices or revisions of the same device can benefit from code changes. Even on the same device unit, they might want to replace the ROM chip to include some fix if it's important enough and makes financial sense. The software itself transcends the constraints of any particular delivery medium.

Ha, perhaps. I work in game dev, and previously had a stint in integrated display controllers for feature phones. Anecdotally, most of my code is effectively thrown away once shipped.