X-ray analysis reveals hidden composition under iconic portrait of Lavoisiers

Generally you are correct, a work unrealized is mostly a question of might have been. This is probably more special than most, as it isn't a hidden work. Instead we are seeing changes made to emphasize a different aspect of the sitters. It is also interesting because of the dual nature of the pair: hated tax collectors and beloved scientist who fundamentally created modern chemistry.

The changes like removing an ornate hat from the lady and emphasizing the scholarly work of Lavosie are interesting as they show the concerns an aristocratic couple had about their public image on the eve of the French revolution.

It is special because the painting itself, and its hidden elements represent a historical turning point.

It shows visually the rise in popularity and importance of natural philosophy. Changing the context of the painting from an aristocratic centered piece to one highlighting scientific endeavors is an important milestone. It was popular before, for this reason, but these added details from the conservation really highlight the changing ideals in Europe at that time.