Spot on.I completely understand this take, but I look at it from a different angle than most. My education and career before taking over my dojang was in science. I see TKD and martial arts as a whole as an academic endeavor similar to science and in active fields of science, you have to constantly stay involved be it through lectures, seminars, further schooling, etc. or your credentials become outdated. Sure you still technically have the PhD from 30 years ago but your understanding and methodology may no longer apply and be less capable than some undergrad students who are up to date. Good examples of this are medicine, astrophysics, environmental sciences, and health sciences.
On the flip side, I also understand that some see it in the military view point or from a historical and cultural perspective where once the accolade is earned, it is kept for life. I don't think this is wrong, it's just not how I personally view rank or the standard I hold myself to. It's also why I have never made a student start over or go down in rank when coming back from years of a hiatus.
In regard to getting old and sick, if you are actively involved in your art in some way then age and sickness will have little impact on your knowledge and skill in your art. There is more to skill than physicality, a large part is understanding. I have met many old masters that are physically incapable of training the same way they once were, but their understanding of the art and how to teach it from a mental standpoint grew greater and continued to grow up until they died because it stayed an active part of their life. If you stop doing anything on a physical and mental level for decades your understanding and overall knowledge of that topic will inevitably dissipate and become less than it was, whereas if you can stay mentally involved you can still grow in your abilities in different ways.
A caveat to this, I think this only applies to rank and not title. For example black belt is a rank, Master is a title. just my opinion again, you earn a title and stays for life but rank is symbolic of what you know and understand currently. To use your military analogy, you can earn the rank and title of Major or Lieutenant in the military and still hold that title, but once you retire and are no longer active duty you can't just walk into a base and start ordering around enlisted soldiers and NCOs even though you still technically have the officer's title.
I also have an engineering background (muitiple degrees), and it is definitely a progressive field. Try to set on your laurels, whatever degree level you have, and you will get left in the dust.
I have a feeling I know where you are coming from. The ever-reducing sliding scale of time to black belt and the deepening idea that a 1st Dan is just a 'serious beginner' is beyond maddening at times. People getting a black belt a year or so, then stopping, then starting back and feeling they are 'earned' much of anything is a very slippery slope that TKD is headed down. The automatic expectation to be held at the same level after a long break in something as physical as a martial art just doesn't make much sense on some fronts. But it is the sage person who realizes that ability in the martial arts has as much to do with our mind as it does our body. So, the value of training does not start to reach critical mass until 3-years or so of regular training, IMHO. In this context, it does hold enough, at least subjective value, to be regarded similar to a college degree or equivalent.
Being involved in multiple styles of TKD for a Long time, I have seen more than a few black belts come and go. Some, really good physically/competitively, some, really good mentally, and a precious few who are/were both. Some who could care less if you acknowledged their rank, and some who care far too much that theirs be acknowledged. Most land somewhere in the middle and that is okay as long as they understand that whether we are at the age to realize it or not, a paradigm shift will happen to our body and possibly our mind at some point, leaving us less than we once were. Holding on to this so hard it gets in the way of passing on our knowledge to someone else it the worst thing any of us can do.