I have a different issue. Everything kindof always gets promoted to a must have. So now I've made 2 priorities, need-to-have and nice-to-have. For any given collection of work that needs to be done there can be no more than 60% need-to-have estimated work hours. When the deadline is accepted by my team, it means that all need-to-have are done at that deadline, and nice-to-have probably are, but no gurantees.

Every single fucking time the PO thinks the deadline is too late, she tries to remove nice-to-have issues - sorry, but that doesn't change the deadline.