--snip--
Allow me to answer your question with a question. Strictly speaking, Bitcoin can be "printed" by changing just one line of its source code. And if 51% of miners reach a consensus around this version of Bitcoin, we could indeed increase the maximum supply to 30 million or 50 million. So, why is the number 21 million significant? Is it some kind of religious belief?
/** The amount of satoshis in one BTC. */
static constexpr CAmount COIN = 100000000;
/** No amount larger than this (in satoshi) is valid.
*
* Note that this constant is *not* the total money supply, which in Bitcoin
* currently happens to be less than 21,000,000 BTC for various reasons, but
* rather a sanity check. As this sanity check is used by consensus-critical
* validation code, the exact value of the MAX_MONEY constant is consensus
* critical; in unusual circumstances like a(nother) overflow bug that allowed
* for the creation of coins out of thin air modification could lead to a fork.
* */
static constexpr CAmount MAX_MONEY = 21000000 * COIN;
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.