If one day bitcoin is gone I'm sure people would lose trust towards crypto currency as a whole. Besides at this point I really don't see an alternative coins will be worth taking over its place.
Thats bad because Litecoin can take over anytime, its almost a clone of bitcoin, so it would be easy to take over.
Plus by then we would have more experience with cryptocurrencies, so obviously any error that bitcoin had, would be corrected in the others.
Besides dont forget: NXT, NMC, PRC, GEMZ, BYTE,etc. those other altcoins will be worth something without BTC aswell. Not to mention ETHER, and RIPPLE.