I think it will survive until people believes in the technology. Since bitcoins can last indefinitely in theory, bitcoins will be relevant if there are people that are willing to buy and sell it. The 21,000,000 bitcoin cap is just the number of bitcoins that will go out in circulation. The part of mined block, is just actually rewards. Miners can still continue mining blocks even without rewards. I guess that's a good thing for us, since our transactions will still be written in the blockchain. But I don't think I'll live to see that day since it's so far away.