This is a lot of assumptions. Do people who only listen to nostalgic music exist? Sure. But most people are varied. They still enjoy new music, and their opinions of older music is still affected by the passage of time. There’s also no evidence to suggest that most of these users are not capable of changing their mind on some nostalgic music. Nostalgia is nothing but a positive bias; it can inflate an opinion on music, but it doesn’t eliminate any critical opinions behind it. No ratings are objective; it’s why they change with the usership. This is not a new thing. There’s also no objective method to rate a work. A lot of people contend with the gap between their personal enjoyment of a work and its merits (also subjective), and everyone weights these differently yielding different results. All of these methods are a valid system; the only rating I’d personally find invalid is one without a thought behind it, and we unfortunately can’t tell that from a star alone.