Cavaliers are such stoic little dogs. They can tolerate a great deal of pain or discomfort without showing any outward signs. The fact that your Cavalier fails to show outward signs of pain on a daily basis, does not mean that she is not suffering from it.
My boy Rebel was diagnosed almost 6 years ago, without any symptoms, when I took him for a routine MRI scan. I was given prints of the scan, which I showed to my vet, who made a note of his evaluation. A couple of years later he did show signs of severe pain, so we went back to the vet so that he could be prescribed the appropriate medicines to relieve the symptoms. He has progressed up the scale since then, because this is a progressive disease for which there are treatments to relieve the symptoms, but not the disease, as you asked.
I would say yes there is a very definite point in treatment for the symptoms and not the disease. I have the pleasure of being with and sharing love and companionship with Rebel, despite the treatment he is receiving and the adjustments I need to make to his medications when pain becomes severe. These episodes are short lived and very few and far between I must say, so I do not feel guilty or selfish in continuing with treatment for what is after all an incurable and progressive disease.
Rebel is one of my four Cavaliers of both sexes and varying ages, so please do not think I am being selfish or clutching at straws by treating what you may see as a hopeless cause because you think I need a dog to love, no matter what the price. This is very far from the truth and I think you should start to think the same way. You have a dog as a pet to love and hopefully to be part of your family and lifestyle, which all my dogs over the past 40 years or so have been. If one of those dogs presents with a disease or injury which is terminal and causing undue suffering, then the decision has to be made to euthenase for the good of the dog. If the dog presents with something progressive but with controllable symptoms, then you utilise all the skills and expertise of the veterinarians to give that dog a comfortable and happy life until the time comes when he either dies or his condition becomes untreatable.
Please do not think that I am criticising you in any way or talking down to you. Far from it. I am just trying to rationalise all that needs to be considered when facing a disease such as SM.
By the way, I have had Cavaliers for 31 years now, usually in fours or fives at any one time. Several years ago I bred and kept a bitch who I think showed symptoms of SM at round about 13 years of age. Nobody had ever heard of SM at that time and some six months later she had poor quality of life so was euthanased. The only other dog I have had in all that time, including those of the 8 litters I bred, only Rebel has been diagnosed with SM. I prefer to remain optimistic about this disease and hope that the longer research continues the earlier either a retardent or a cure will be found, not to mention a genetic test for breeding stock to prevent sufferers ever being born.