It becomes useless when those doing it cannot clearly articulate how it helps.
From Thornton's standpoint and criteria, he is looking for someone that does the "old things" to show him how it is useful. It appears that no one has been able to do this. So you can't fault him on that.
"Old" is useful to me in many respects, it depends on the context of what I am training.
Again, I think it is important to understand and be able to articulate how "Old" is useful.
Most folks in TMA cannot though, and frankly, I have abandoned much of my TMA training as it is simply NOT an efficient way to train for the things I think are important to me.
The day someone can demonstrate to me that they are...then I will do them.
There are a number of things I do on a daily basis that are considered "Old" and are considered basic "kata" exercises.
I think the difference is...I view them as exercises and structure, and NOT methods of fighting.
I agree with you. If you don't have someone who can explain what is the meaning of doing some or other form, then it's useless. Unless you are clever enough to pick the things that are "Hidden in plain sight"
(which is not my case btw
I've seen 3 or more different bunkai to the same Karate kata depending on the style (and even the school inside the same style!). And the same movement was slightly different because of the application they inherited or they found to be more likely (easiest way to loose the origins of the art sometimes). What was the original idea behind it? Who knows, maybe the person who did the original kata died with the secret 100 or more years ago.
So the problem is always the same. Finding a good teacher who knows the "real deal". Will that be possible in 20/30 years?
But as David said before, I think that old forms still have utility, whether you use them as a key to understand the art, or as a mean to gain a good structure as you said.