Bcash cost of production (difficulty) is now being slashed by 20% per block for the next 6 blocks. Expect miners to follow cost of production slavishly. Forking guys have no idea what they have unleashed ... it will become apparent soon now.
Yes, the existence of BCH will destabilize BCT mining.
Everyone knows that at equilibrium, if BCH has 1/10th the value of BTC, that the hash power mining BCH will be 1/10th that mining BTC.
What is less obvious what happens when the system is not in equilibrium. Imagine that the market reprices BCH 100% higher and BTC 10% lower. What is the expected distribution of hash power now assuming short-term profit-maximizing miners?
The answer is "most of it will be mining BCH." Because the difficulty adjusts only very slowly (every 2016 blocks) BCH becomes twice as profitable to mine as BTC. Hash-per-hash miners would earn double by mining BCH. This continues until the difficulty reset comes when BCH would go "limit up" (4X), when most miners would leave BCH back for BTC. BCH's difficulty would slowly ratchet back down due to its fast difficulty adjustment and the process would later repeat.
What this suggest is that at times when BCH is more profitable to mine and the hash rate migrates to BCH, the average block time for BTC will increase significantly and BTC's already slow and expensive transactions will become much more so.
What do you know, Marcus, it worked.