Customers who wish to access both bitcoin (BTC) and bitcoin cash (BCC) need to withdraw bitcoin stored on Coinbase before 11.59 pm PT July 31, 2017.
This applies to all online wallets and exchanges. You should have all Bitcoins in your own off-line wallet before the fork. Then your coins are available on both blockchains.
Look what happened when Etherium forked. Etherium Classic is still around and tradable. It dropped, but not to zero. Why give up that value? Bitcoin Cash might get some traction. If not, you haven't lost anything.