Yes, but that's not the main issue -- it's that thousands (millions?) of other bulls who were around during the 50's and 60's didn't get to contribute to the ge

Yes, but that's not the main issue -- it's that thousands (millions?) of other bulls who were around during the 50's and 60's didn't get to contribute to the gene pool leading to low genetic diversity. If there was some hidden defect in the Y chromosome that happen to affect both of them, that defect would not be in most bulls in existence.