Why would Bitmain shut down their customers' hardware? It's a sure way to kill their (quite successful) brand.
But even if it happens, all you have to do is update the firmware again, and keep mining.
But let's imagine someone actually does shut down 70% of the mining power. What's the real consequence? Blocks will be mined somewhat slower (but not even close to 3 times slower) till the difficulty self-corrects. In the very worst case that will take 2016 blocks, or 14 days.
But even if it happens, all you have to do is update the firmware again, and keep mining.
But let's imagine someone actually does shut down 70% of the mining power. What's the real consequence? Blocks will be mined somewhat slower (but not even close to 3 times slower) till the difficulty self-corrects. In the very worst case that will take 2016 blocks, or 14 days.