Won't be for a while yet the BCH chain is ran over 90% by just one miner and the BCH chain is creating lots to small fee transactions on BTC making the mempool artificially fill
Right now a few big miners on BTC could collaborate and wipe out the BTC chain within a day and if the attacks carry on it could be possible
Well, there are also the wonderful market forces that Satoshi considered while creating Bitcoin. Should some miners leave the BTC chain, I am sure others will pop up as they consider that an opportunity for gaining BTC with lower efforts to enter the mining process and actually compete from a hashing power perspective.