I doubt the #2 coin based on market cap will vanish any time soon. There would have to be a monumental failure for such a thing to occur. Even then, it is likely to have code forked and altered from a state from before, giving it continued life.
Even if ethereum were to disappear, something that is incredibly unlikely, we still have ethereum classic which is basically the same with the difference that led to its creation which is that it respected the immutability of the blockchain something that I agree with, and as soon as ethereum disappeared we will see hundreds of forks trying to revive the coin and fix whatever killed ethereum.