Because of some of the changes made to villagers within the 1.14.4 pre-releases, villager "demand" nbt will never go down on its own.  In previous versions, demand would slowly go down whenever a villager would work at their workstations, regardless of being traded with.  Since villager demand only goes down when a villager "restocks" a villager's demand will never go down unless one of its trades is locked out and it restocks its trades.