the monopoly on violence is only possible because the military institutions are funded by an infinite amount of unbacked money. If bitcoin is the only money people want to be paid in (see romans only wanting true gold) then the military institutions will need to create value for their country otherwise there will not be enough money to fund them (no one will want to pay bitcoin for a wasted institution)

...so when bitcoin is ubiquitous then there may still be security forces but they will be paid for a specific service and the concept of the standing army will disappear because it is too expensive.