There doesn't need to be a consensus. Just take a look at the recent forks. It just takes a faction to decide they want to change the code and they can break away. Then if the original Bitcoin fails because of miner apocalypse and/or compromised security then the ones left standing are the fork who killed the block halving.
100 years is a long time to see something happening. 100 years ago we didn't even have TV, let alone the internet....