> If e.g. Adam Levine were replaced by a humanoid robot or a deepfake, would any of his fans actually notice or care?

As a Hatsune Miku fan... I can say... Yes?

People have been using AI generated vocals for well over a decade. The Hatsune Miku community differentiates songs and can tell who the composers or artists are even if they're all using the same voice.

There's a pretty big difference between say... "Karakuri Pierrot" and "Senbonzakura"