I'm not doubting that at all, but I just don't see what you obviously have seen in regards to the early Basenjis having poor temperaments. Interestingly I recently read an article about someone importing a village Basenji from South Sudan, and "soft temperament and willing to please" was one of the things mentioned. Genetics can certainly play a big part. The old "nature or nurture" argument. I had three foals from the same mare, different sires, and all raised the same by me. Three totally different individuals! My concern is that breeding for one trait may have unintended consequences on another, and breeders do tend to want the lines that are winning.
A dog with a bad temperament isn't going to do much winning. Snap at a judge? Bye bye! Best case the dog is excused for the day. They need to be comfortable with many different people touching them and as I vet tech I can firmly state that most people's pets are NOT comfortable being handled/touched the way a well trained and socialized show dog is. It's true that while working to enhance one trait you can add something undesirable to your lines but temperament should always ALWAYS be high on the list for any responsible breeder's program.
Also, I know a lot of breeders.. they don't always want or like what is currently winning. A lot of the top dogs (in any breed) are only top dogs because they have the $$$ to back them. Not that they're bad dogs and don't deserve to win, but the more a judge sees a dog the more they think it to be correct.