I am hopeful some others will attest to the durability of the No Rez. Mine is not that old and frankly, mine is sealed inside the speaker so I have not had cause to inspect it. Being sealed, it is not subject to exposure to heat, sunlight, moisture, or other factors I suspect may have degraded your panels. As for volume, Danny normally says that volume of the speaker is barely impacted by the no rez as it is mostly air. He will likely tell you that the no Rez does not significantly impact the internal Volume. That is the reason he encourages dampening in addition to the No Rez.
Listening to your responses, I am wondering why you have not commented on how they sound right now? You clearly have some doubts. I am wondering if you are wanting new, or wanting better? NO big deal either way; modified speakers can be like new. But you might be happier with a purchase (whether upgrade or new kit) if you can better isolate your motivations. It seems like you have predetermined that the age is a problem for you, not withstanding what they sound like. From the photo you added I am thinking that you might learn alot about what your vintage speakers can do by moving them away from the walls. They may benefit from experimenting with other amplifiers as well. Even new speakers will benefit from "fitting" them to your room, or suffer from a failure to do so.