It looks like BCH is here to stay. For now, it seems that the combined value of both BTC and BCH is, consistently, around $3000. When BTC goes up by $50, BCH goes down by $50 and vice versa. Not sure how long this relationship will last. I do not mind for my exchange to sell off BCH now in exchange for more BTCs.
They said that Bitmain and Roger Ver were going to sell off their bitcoins and put the money into BCH after the fork and they are probably doing it right now. That is why the price of BCH is sustaining at this moment. Once they run out of the capital from selling bitcoins, the price of BCH will probably drop.
If BCH made it and survived with a decent price, that would encourage more similiar Bitcoin split in the future - which is not a good thing in my humble opinion.
This correlation might be true now but will only be lessened as time goes by because after all, bitcoin cash isn't really anything more than an altcoin airdrop. Bitcoin and Ethereum probably has an inverse relationship as well because there is only this much investor funds available in the crypto scene. Bitcoin cash is not different.
Logically speaking i can see why this correlation is true, so don't get me wrong. Thereotically, yes, bitcoin cash + bitcoin = actual bitcoin value pre-fork. But practically it is quite different. Bitcoin cash has now risen by 70%, whilst bitcoin has only dropped by 3%. If what you said was true, then bitcoin would have dropped by at least 10-15%.