1. Consensus involve all Bitcoiner, not only Bitcoiner who happen to be miner.
2. Even if miners (who have total >51% hash-rate) decide to change total amount of BTC, other people who use BTC would ignore their block by default since it violate existing Bitcoin protocol.
3. IMO number 21 million isn't significant, it's just arbitrary number.
4. The change definitely require more than one line of code, such as re-determining mining reward.