copyright should be ethernal
and belong to owner of art piece or thr rights - only if owner has a contract for copy rights.
if owner has no contract signed by artist on rights for reproduction - the owner can not claim for those rights, because some one else may be their owner ... so, if a Vatican can not present contract for such a rights signed by Michelangelo - those rights are not manageable . - sorry... -and every body can claim his relatives had oral agreement with Michelangelo on these right as well as Vatican... as it hard to imagine Vatican has a writen document to base it's clames - they actualy will lose in every court claiming for those rights... as for restoration - cleaning the statue not give an owner of statue or cleaning stuff rights on photos of cleaned statue - the rights are of photographer, and and he has to have aproval from owner to make photos, but not the reproduction of photos - because owner - may have no right for reproduction, so he just can not sell rights he doesn't own. If owner is also a photographer - he can claim for rights on photos, so actualy if you can prevent from other photographers to make photos of something - you can own reproduction rights forever... - and i sugest to vatican to cover with curtains the ceilings of capela