due to decentralized nature of digital currencies, there is no significant upfront cost for a government to switch to digital currencies.. physical money needs constant printing and monitoring.. but bitcoin is financially and environmentally friendly.. even though it has more advantages, replacing money by bitcoin is a doubted one....
you need miners anyway to sustain the network, so it does not come without cost, and to secure a new currency you need a massive hash, so don't compare it to the past history of bitcoin