You need a consensus about what software to run, what it does, what version, and you need the consensus of the community to run nodes and mine.
Imagine that a large majority of miners bet the wrong way on Bitcoin at the CME and go in hoc up to their eyeballs short BTC. Let's say 80% of the BTC mining community is short big and they need BTC for delivery now. As a general proposition, that community could reach a consensus to modify the software they are running to inflate away their debt. The consensus is just like any other fiat currency, and it is very unlike the core nature of gold. The very scarcity is determined by consensus.