I understand what you are saying. I would have said the same thing too in the past. However there is no proof that we can link that here has been a change in temperament. There are changes. But the changes lies in how the dogs are raised. conditioned, etc. The argument I hear all of the time from those that are anti show is that a show dog won't bite or can't do the work. These people do not breed to the standard. They breed "working" dogs.Often they know the dogs they are breed do not meet the standards set by the parent organization. If temperament was genetic link every dog from the working line should make great working dogs. They should all be able to earn a schutzhund title coming from strong schutzhund lines. The fact are that every dog is not going to be a great schutzhund dog just like every dog from a "show" line will be champion of record. The true difference is how the dogs are being raised. Today dogs are being raised to be more socially acceptable. They do more early socialization to allow the dogs to be touch and handled by people other than the owners. I recall when we were taught not to let anyone touch your dogs. In fact they taught you to raise the dog in a crate and to not let in see any strangers or have any contact with humans outside of the family. The bite development started at 8 month. All of the puppy imprinting was geared towards developing the dog's drive. Some still hold fast to this type of methodology. However, today we are taught the dog should see over 1000 people before it is 8 weeks of age. Many look down on those of us that use forced correction. They don't even believe dogs are pack animal. There is no alpha(mentality) type dog. Some say there is no different drives like prey drive, etc. This they back up with scientific data. Many believe if we use a slip collar or prong collar that is abuse. They try to solve everything with cookies and hot dogs. I know a ball can be used instead of food for some dogs. However edible are the most common way to reward the dog positively. Of course a dog that has been trained from the time it has been weaned to to guardian type work are going to be better at that work than a dog that has been taught to be socially acceptable. But this does not change the instinct that the dog has to to guardian work. The instinct is there. But it has been taught what its job is. It can be taught through training. It can also be taught by seeing other dogs. Often my dogs learned what to do by seeing my other dogs do it.
Science has concluded that temperament has a genetic component. But it is greatly influenced by environmental conditions. It is developed by experiences. It is polygenic. There are multiple or many things that influence temperament. Scientist have been studying dogs behavior or phenotype. But it is difficult to defining behavioral metrics. It s extremely complex. They can draw conclusion, but there are also many independents variable that they can't explain. If it were all genetic every dog in a littler would be champion of record. Every dog in a schutzhund line would earn a schutzhund III title. We know this does happen. There are still going to be some pet quality dogs in a litter. You are lucky if you get 2 or 3 goo show quality dogs. A schutzhund line might produce some good personal protection dogs, but in that same litter not every dog will be able to handle being a good schutzhund dog.
I agree that there has been changes in the dogs. It is related to AKC and other breed registries. I do not fault the registries, but IMO it is directly related to them. Since the registries accepts breeding dogs from anyone with a dog we have dogs that are not breeding quality being bred. People say they want to improve the breed. But they only want to create their ideal dog not the ideal dog for that breed. They want a bigger head, a larger dog. We can look at the Huge dog that is being talked about all over the internet right now in one other than Hulk the "biggest pit bull" in the world. It is a beautiful dog. It is impressive looking. However a pit bull is not a 175 lb dog. Of course I wasn't there when the dog was bred or the dogs that produce this dog was bred. But somewhere along the line it would surprise me if something else was bred into it to get the size other than pit bulls. I wouldn't be surprised if someone hung some papers saying this was a purebred. Since there is no score that a dog has to reach before it can be bred and its puppies get registered anyone with a dog in their yard can breed. The lack of selective breeding practices have produce the changes and health problems in dog breeds.
If we go back to selective breeding the ideal working dog or dog in the Guardian group should have the correct conformation to do the work as well as the correct temperament to carry out the work. If the dogs do not have both they should not be bred IMO. That leads me to the policing of the breeds. This says to me that this is up to the individual breeders since no registry make this a requirement.
There is this "breeder" in Canada. He has presa canarios. His thing is size. He want all of his dogs to be huge. In the process of doing this his dog lose agility. Allegedly his top stud is huge and can't jump into the back of a truck. It has to be picked up. Sure it's huge and looks nice. Some would say impressive. But it is no functional. I was at a big presa event. There was a dog that wasn't fully matured yet. It was 148 lbs and still growing. He can still move gratefully. it was doing a temperament test (in defense of handler). It performed well. It hit the sleeve hard. Might have been the hardest hit if the day that I witnessed. I didn't see them all. But the problem I had was it took a long time for the dog to get to the decoy. I could have taken a nap before it got there. It was too big. Again it is not functional. Hopefully my dog will never have to protect me. But if it does I need to know it will get there fast. I need to know it can go over the fence and stop and attack if needed. Time can be the difference in me surviving or not.