I agree that every dog will not make a good protection dog even breeds that are supposed to be protection breeds. I differ on the opinion that the breeder should know the difference in pet quality and guard dog quality.
Pet quality is a dog that has too many faults or disqualifying faults. This is related to conformation and a dog that is too shy. A dog that is pet quality can also be a great protection dog.
Any time you get a pup you are taking a gamble. There's no fool proof method to determine if a pup will be a champion of record, Schutzhund, IPO, French Ring or personal protection dog. We do know certain breeds are excellent in these types of work. Certain lines exvel, but can and will produce a dog that is not the best suited for the work. There's some puppy implanting that can be done to increase the probability that the dog will be a good worker with solid nerves. But as with every dog things can change as the dog grows and matures. There are environmental things that can affect the dog and cause the dog not to perform the best.
Considering your 8 year old dog without evaluating it personally it is possible that that dog will protect. I'm basing this on your statement of its defense with a trainer. With more training and confidence building it might be possible. Some dogs take to it right away. Others require more training.