You're hitting on a critical point about cryptocurrencies that isn't widely understood yet.

Ultimately, the community of users and businesses at large decides. Cryptocurrencies are social contracts that are encoded in software. If everyone agrees that a rule should change, and updates their software in concert, then the rule de-facto changes.

In the absence of total agreement, the decision is made by whoever wins the political fight for user/business mindshare. It's also possible for the question to be answered two different ways (a longstanding fork occurs).