Water vapour from within a house does not go through the wall. It goes as far as the plaster, maybe a bit into the wall, and then it evaporates back out into the room air. There is not enough pressure to push condensed air moisture any further. Other moisture is held within the masonry in a state of equilibrium - controlled (or managed) by the structure and ambient air movement.
Insulation board will only break down when in conditions that attack the material structure - UV or a chemical for instance. Built into a wall, the material lasts as long as the building.