In normal circumstances the value of a spin off should be subtracted from the Bitcoin value. The reason why it did not happen could be that the Bitcoin market is not efficient or that Bitcoin cash really creates value. Both are reasonable explanations.
I think Bitcoin Cash is to stay since all Bitcoin owners did receive a free lunch and are held hostage by Greed and thus will try to maximise the profit.
I am only afraid that a new hardfork might be coming and that we will face three Bitcoins at the end of the year.
In case all hardforks will create and not destroy market value it might be a good thing although I would be suprised if the world really needs three types of Bitcoins