I have not been able to read all the way through this but I have trimmed my horses for years and ridden to A levels and now do mostly dressage - compete- and my horses are barefoot. I am not going to criticize shoers or shoeing. I have one rescue horse with front shoes on right now - he came in with all 4 shoes and his feet are very soft. I can’t keep him - we have to find a home for him - he is at an EQ center looking for a home. The farrier I use on him I LOVE dearly and he is terrific. And he actually helps trim some of my horses at home because I do not have the back for it.
From everything I have read about navicular and experienced (which I work with hundreds of rescue TBs and I see a lot of cases - also we have had several in the family - AND two of my close friends had horses with navicular).
Firstly - ‘navicular’ is a bone and in layman’s terms - a horse gets navicular pain when a lack of blood flow within the area make the bone get spiney teeth on them - this is a very layman’s description - but it is sometimes the best way to explain it. For years past, it was believed this was a disease - something a horse has perhaps in his genes. In the years recent, they separated it between Navicular Disease and Navicular Syndrome. The Syndrome is when the conditions are right for the condition to occur. The conditions might be poor shoeing - shoeing too tightly and restricting blood flow. Arguementably, some believe that shoeing in general can cause the conditions (although we know that not all shod horses get navicular pain… so that can’t be true - and I am a barefooter). However, I do think that a lot of horses that have the pain got it from shoeing - perhaps because other factors like use and conformation also play into it.
That said - I have a few friends with horses that developed navicular pain before their horses were 5 and never been shod. Ironically, they were ALL color-bred Quarter Horses, 2 paints and a palomino. All three were huge boned and over 16 hands. The third one - when I saw her - I knew she woudl get the pain. She just looked conformationally exactly like the first two. I do not think with them it was a DISEASE but I certainly think their big bodies and their tiny legs contributed to it.
Anyways - I think also it has to do with use - a horse in shoes used as a jumper and to do things that cause a lot of concussion - those horses I see get the pain in their teens. When I grew up we pulled the shoes on horses in the winter - let their feet grow out and then shod through show season. None of us ever encountered navicular pain.
Anyways - the original thread was - is there hope? I hate to give false hope but there is a lot out there by way of research lately - drugs to try to help break down those ‘teeth’ - methods to stimulate the blood flow to try to get the condition to reverse - which some people scoff at but I have seen it happen - usually not in cases that have gone as far as yours though. Usually in horses with a little pain and a certain degree of change to get worried - and then with different therapies - like, yes, barefooting - the condition did reverse.
My friend who is a shoer - he admits that bar shoes and the like - that is one of the best ways shoeing can help a horse - it helps to relieve pain - but if a horse is having the problem to begin with because he needs to go barefoot and let his feet spread and breathe and pump more blood into it - the condition will get worse - the shoeing will just help control the pain for a short while and then you find yourself where you are - where the shoeing that way doesn’t help.
I think it is a personal decision what you do. I think you are looking closer at a better chance at a remedy or at least a way to get him better - by doing barefoot therapy. That is my personal opinion.
But I am not blaming shoeing as the reason for navicular pain - if that were true - most horses would have it. I think there are loads of conditions that play into it - a horse that works hard - jumping, reining, whatever. A horse that works on hard ground a lot. A horse that is physically conformated to be predisposed to it under just a few circumstances. A horse shod and kept in too small of shoes. A horse perhaps always shod every day of his life. A horse where his owner keeps forgetting her checkbook so he goes 10-12 weeks between shoeing on a regular basis…
I think that the pain comes from growth on the navicular bone that breaks into soft tissue and causes pain. Those growths are made by a lack of blood flow and health within the foot. I just wrote a bunch of conditions that can contribute. I think to heal it - if it is possible - is to look at the way the horse is worked and kept and try to eliminate the condition that is causing it. That is why I advocate at least TRYING barefoot therapy. But then, the reason I say it has to be a personal decision is - the horse will go through a period of discomfort. The bar shoes do take weight off the spot where they get pain when stepping there. Some people say - my horse is too old and doesn’t do anything but mosey so I won’t put him through the transition period to barefoot therapy… I can understand that.
And that said - the three horses I know that had it before age 5 and never shod (well one had been shod) - they went through barefoot therapy and it did not help them - their bodies were too damn big for their legs and I think their conformation just wronged them.
Two of these were mares and they went to the breeding shed. Oh my word.
I spent a great deal of time earlier this afternoon on the phone with the vet, and many of the factors point to coffin joint problems, so reducing concussion, which I understand a lighter shoe can help with, could be the one important shoeing change we could make. I also was told recently that an aluminum shoe can be a good choice for a horse with thin walls, also due to the lighter weight.
It just goes to show you that nothing on these beasts is ever static, which is why I really monitor them like I do – and why I ended up without the funds to take that manure spreader off your hands! 
I’ve learned that the downside to catching every little problem is that it can be expensive 