This has a nice, Borgesian, charm to it but I feel like it's premise is confused. If it's just about the here near-empty signifier of "happiness," what does the magic tech of the earring actually contribute to the parable that some hypothetically perfect narcotic wouldn't?

The earring doesn't make you feel better, it actually produces a better result. It's never wrong. You might regret addiction, for example? Also, it's a parable. Take it too literally and it loses its charm.