If that ever happened, which I seriously doubt, price would tank much more than 50%. The panic and the confusion could be catastrophic for people's trust that their currency is worth what they paid for, trust is by far the most important asset for BTC. Once it's gone...we would see at least 75% price loss, I'm sure.
Gaining trust is not that simple. If bucoin gets mined continuously too it won't gain the trust from the digital currency users. Several coins are getting listed into the exchanges regularly. As a part bucoin gets added. On basis of this there is no need of panic or losing trust over bitcoin. Op has exaggerated the antpool mining in comparison to segwit that's it.