If Bitcoin becomes the reserve currency, the theory goes that the US would not need to involve itself in as many petrodollar wars as it does today, thus resulting in a smaller military.
I don't think anyone seriously thinks Bitcoin will replace militaries completely.
Wouldn't the US become more desperate to hold onto its influence in oil producing regions and do so with violence? don't see why "wow, everybody is starting to use BTC rather than USD for oil transactions" would lead to "guess we don't need this aircraft carrier" instead of "time for some coups to install governments who will do what we want and not switch to BTC".
The petrodollar is basically a conspiracy theory - you can use dollars to buy oil priced in Euros the same way you can use Bitcoin to buy drugs priced in dollars. Iraq pricing oil in Euros would just mean they sold most of it to countries using the Euro.
The importance of the role of the dollar as a reserve currency is also just an urban myth:
I don't think anyone seriously thinks Bitcoin will replace militaries completely.