Bitcoin maximalism is descriptive. It is the conclusion you reach after understanding monetary economics and the nature of Bitcoin. Markets have a strong incentives to select for a single currency and Bitcoin has numerous advantages over altcoins (first mover advantage, decentralized leadership, and lack of hard-forks in particular).

Bitcoin maximalism is not prescriptive. We're not saying that the world ought to have one currency or that bitcoin ought to be adopted worldwide (at least not for reasons following not from the descriptive arguments).

Bitcoin maximalists are simple advocates of the first position, based on it's logical reasoning.