.. but the meaning of the blockchain and its purchasing power (or other real-world effects) are entirely determined by the software that interprets it. We've seen this a number of times, such as the Ethereum "DAO" roll back.

> Miners make decisions on which fork they want to run.

Miners are surprisingly consolidated, so this mostly determines which version "wins". And the economic value of which fork wins is determined by the markets. If you keep running the "wrong" version of a fork you find out that it's worthless and only the coins on the "right" one retain value.