You're absolutely right wvasko.
A reactive pup showing defense at 6 months old isn't necessarily a lost cause, but it's definitely a dog that would require an experienced handler and an even better trainer. From what I have seen however, most puppies displaying this type of tempermant at 6 months old have some underlying issues that present themselves when they're a bit older that tend to exclude them from defense training (most commonly, a bit thin nerved or too sharp). I have seen some solid dogs that were intense from the get-go, so you're right, it does happen. Just not that often. I like to see a happy, confident puppy at 6 months that eagerly approaches all within it's sight if given the chance, full of gusto and enthusiasm.
I love the puppy stage!