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Your missing that three people(four now i believe) have been able to do it, and the cost of it is too great(AKA blindness) for it to be used as many times as it would take in order to kill byakugan user.you don't want to miss the point that Mangekyou Sharingan is only capable of being earned by a person who uses the sharingon allowing them to use Tsukuyomi or Amaterasu.
Also if need by relate sasuke and Neiji in a fight w/o using jutsu except for their bloodline traits.
The Byakugan's 360 degree vision and insight renders high speed inefective, because the Byakugan can see, sense, and evade movement within thier field of vision near instantaniously. In the instance where the Byakugan user does not have the reaction time to evade, they can merely use the Hakkesho Kaiten to knock the attacker away to prevent them from getting near