Whatever the morality involved, it's an unstable norm. There can be good actors who use AI and disclose, and good actors who don't use AI and don't disclose. There are bad actors, however, who will use AI and not disclose. The entire discourse will be flooded with non-disclosers accusing each other of AI use (particularly, the bad actors will accuse everyone else of AI use, to flood the zone with shit), and there will be no way for a consumer to distinguish good from bad. Bad actors will prevail economically.
See e.g. corporate green and rainbow washing, Amazon marketplace, organic food.
I agree that the current culture of disclosure is unstable. Nevertheless, I think that there will still eventually be a mechanism to verify the degree and manner to which AI was involved in the production of art, though perhaps with very different economic scale and distribution channels. Speculating, perhaps there may be an "organic food" niche of mass media art.
See e.g. corporate green and rainbow washing, Amazon marketplace, organic food.