Well, there are a couple of things to keep in mind. 1st of all is that researchers already know that CM/SM in Cavaliers and other dogs is not exactly the same as Chiari and SM in humans. In dogs, the malformation alone only rarely seems to cause symptoms, whereas in humans, Chiari alone is by far, the principal cause of symptoms which can be worsened by the development of SM on top of Chiari malformation. The malformation is also different in dogs, which is why many of the canine researchers agreed to use the term Chiari-like malformation. The human brain is also different in structure to a dog brain so the anatomy of cerebellar herniation is a bit different (dogs do not have cerebellar tonsils, which are the part of the brain that herniates in humans). There are different types of Chiari malformation in humans, whereas there is just one in dogs. Here's some more information and images of Chiari malformation in humans:
http://emedicine.medscape.com/article/406849-overview
Also, dogs walk on 4 legs and carry their head and neck horizontally, whereas humans walk upright and carry their head and neck upright. This difference seems to contribute to some differences in response to surgery between humans and dogs but is not very well understood.
On this article in particular, I think what the writer is saying but perhaps not fully understanding, is that this child has begun to develop SM as well as Chiari malformation, and that is why the doctors wish to operate.
On deterioration after surgery–in dogs as well as humans, the surgery only seems to stop problems for a time and it is very typical for humans to need decompression surgery more than once–so yes, the article is noting that he will almost certainly continue to deteriorate over his lifetime. But you also have to remember that humans live 8 times longer on average than the average Cavalier–so there is much more time for problems to develop in humans over time. Still, some level of deterioration was seen in at least half of the dogs in a follow-up group for decompression surgery in an informal survey written up as a paper by Clare Rusbridge. I am very skeptical that the mesh surgery will have much better results when looked at over a long period of time–e.g., the lifetime of the average dog. That said, “deterioration” can mean many things from a return of some mild scratching on down to the return of severe pain that ends up with the dog needing to be euthanized. I think very few of us would consider the surgery a failure if a severely affected dog that was experiencing significant pain ends up after surgery meeting gabapentin for some mild scratching. Most of us would consider that a successful outcome over the life of the dog. Clare's paper found that more dogs treated with medications alone ended up being euthanized but that many with decompression also were euthanized for pain eventually, too.
But as the article notes with humans–as is also true for dogs–surgery is not a cure. It is an alternative that can give dogs that otherwise might have a very short lifespans, a medium to full term lifespan instead and many people will feel it is worth doing for that reason. Others will feel the risk of failure and the stress of the procedure or the cost of the procedure or the already severe symptoms of the dog mean surgery is not an option for their dog. It is a very personal decision, but any one opting for surgery needs to be aware that they need to take any claims that surgery will leave dogs medication free for life, or free of all complications, is at this point IMHO nonsensical. Data on the surgeries generally is very thin and in some cases nonexistent for anything further than a couple of years of postoperative evaluation. And, too, some of the claims do not fit with what I see people writing about on discussion boards or hear back from people who have had the various surgeries.
That is why I have such a degree of skepticism over claims about one technique being significantly better or having medication free results for dogs. This skepticism has been increased by seeing researchers who initially made these claims, revise them over time to concede that they are having higher proportions of dogs needing medication etc (to me it always seemed very premature to make such claims based on only about a year of postop evaluation). I also am aware of a couple of dogs that have had serious problems or significant deterioration after the mesh surgery, for example, in these do not seem to have been included in research reports. Almost every dog I know of that has had the mesh surgery is on some form of medication afterwards, generally gabapentin but in some cases, an array of painkillers as well–and yet the official figures are that the number needing medication is trivial and I just don't understand how there can be this variation. perhaps it is a case that it is mainly the people who have problems that highlight them, but I honestly don't think that this is the case as I've come across so many people over the years who have had a range of these surgeries done on their dogs. I personally believe that many owners whose dogs end up with difficulties post surgery, or which go back on medications, do not report these things back to the original neurologist (for any range of reasons, but this would not be uncommon, just as most people do not return to their breeder to inform them of serious health problems their dogs have also for a range of complex reasons). So therefore, a lot of dogs that go on to have problems, might not be included in a researcher's small, postop research sample Or even inform their general notion of how successful a procedure is. I think it is really important for people who have had surgeries to keep their neurologist informed on any need to return to using medications or to report any deterioration so that statistics better reflect real-world results.