Earlier Ghash had also achieved this magic number of network hashrate and they had asked miners to move their resources into different mining pools to maintain the balance. However, the case is not the same for bitmain. But I doubt they will be performing 51% attack on the network because it would halt the entire network. That will not be profitable for them in any way. 

Also like Franky correctly mentioned that changing the algorithm will cause a serious security risk so that route is mostly unlikely. Lets just hope for good because that is the only thing we can do now.