Currencies can continue quite happily without notable military power in the absence of outside threats, whilst an absence of military in the presence of invaders tends to hinder taxpayers' ability to create value regardless of what currency they were paid in before the invasion.

Arguing that military exists for the purpose of protecting currency is akin to arguing that cryptographers exists for the purpose of protecting currency. Technically, that is one of the things they do [for cryptocurrencies and banks alike], but it'd be fairly ridiculous to claim that making cryptographers more useful was some sort of negative side effect of currency comparable to Bitcoin's energy use.