[ya_throw]

Who gets to decide the "legitimacy" of a chain? Some document, or the participants?

[betwixthewires]

Legitimacy is determined in the eyes of the userbase, in which software they decide to use and what ledger they regard as the canonical history of the network, i.e. what blockchain they sync. Whichever "Ethereum" the majority of users use is the "Ethereum."

[immibis]

The users, really.

[delusional]

Specifically, every single user has to make that decision individually.