I think there are good chances that a significant portion of Bitcoin network hashpower will go to Bitcoin Cash network. Right now it is much more profitable to mine BCH, but if this trend continues the difficulti on BCH will increase and the miners will have to decide which way to go and most of them will definately choose BTC so in the long term i don't think it poses a threat.
Another aspect would be the 8Mb blocks. I may be wrong, but it is not so easy right now to propagate a 8MB block which is full with 8 MB of transactions . I think it would take few seconds to propagate from a peer to another with average internet connection. Not all the people have 100 Mbps link not to mention 1Gbps.