I don't think "golden age" implies no possibility of improvement. It could improve to a "platinum age", which is popularly thought of as more valuable that gold

I don't think "golden age" implies no possibility of improvement. It could improve to a "platinum age", which is popularly thought of as more valuable that gold (even though it currently isn't). This is especially relevant to music, because RIAA sales certifications are well known, and a "platinum album" has sold more than a "gold album". Or if you want something based on real word metal prices, it could be a "rhodium age".