However, if some foreign qualifications might be seen to lose their validity or worth in time, than their NZ equivalents would have to, too.
No, you've taken that too far. An IQA application which includes an Appendix B request for NZQA to check if the content (what was studied, how long, and to what depth) is comparable to a current NZ qualification is something which happens when the applicant needs to claim bonus points for their skilled job being on the Long Term Skills List. For instance, here
http://skillshortages.immigration.go...iness-analyst/ is the page for one skilled job, and if you click on the green box for LTSSL, you will see the NZ qualifications, ONE of which your foreign qualification would need to be comparable with if this was your job, and you needed bonus points on your EOI to reach the 140 threshold for selection. So an IQA will say, if all goes well for that applicant, that his degree is at NZ Level 7, and he has a specific outcome that his degree is comparable to (for instance) Bachelor of Arts (Information Science), which is on the LTSSL list.
If someone's degree is once assessed as Level 7, even if the course for the NZ qualification to which it was found comparable stops being offered, that is no disrespect to the foreign qualification, which is and always will be at Level 7 - it's just that the IQA saying it had the comparability required for LTSSL and bonus points is no longer correct.