My understanding is that Yossi attacked because he - rather suddenly- discovered that he was dominant and stronger then the females. He had grown up in the herd and from what I have heard, he was never dominant over the matriarch Atari, and never challenged her. Until the day he did, and that ended deadly for her.
But the breeding of African elephants would have ended soon anyway. In the last years before he killed Atari and was seperated from the female herd, he bred with only 2 females - Norris, who died during the brirth of her 7th calf, and Bahati, who is his own mother and would have stopped breeding anyway due to old age. That she did not have more calves sired by her son is a good thing in my opinion. The other female who is still alive today in Ramat Gan only ever had one calf many years ago and was a nonbreeder, like Atari herself. So the killing of Atari did not really stop the breeding of African elephants in Ramat Gan, mismanagement (sending away all the young females and seperating the matrilines) and the failure to exchange the bull was.