Are we absolutely certain any CBDCs will involve blockchains? In a centralised currency, blockchains have notably few real benefits over simply using an ordinary database.
I'm sure while they're in the current research stage, while different countries are still looking into what's possible, I've no doubt they'll consider blockchains, but may conclude they're not the most efficient option.
That depends. If governments want security/reliability above anything else, they can use existent centralized blockchains like XRP or IOTA. But since CBDCs will be utterly centralized, a traditional database might be able to serve their purpose well. After all, Blockchain is all about decentralization eliminating the middleman from the process. If the ledger is centralized, it cannot be considered a Blockchain. Due to CBDC's centralized model, they'll enjoy of greater performance and efficiency than traditional cryptocurrencies such as Bitcoin or Ethereum. Banks will win in the long run, dominating the world's economy as they've always done. I believe that the process of launching CBDCs to the public will be much quicker than we've expected.
As far as I know, developing a network from scratch with specifications for the CBDCs is not an easy task. This may take quite a long time to materialize as developers need to thoroughly test the new digital currency before it's launched by the central bank. If governments and central banks are beginning to plan how CBDCs will work and how will they behave in the mainstream world, then it'll be decades from now before physical cash will be phased completely from circulation. For what I know, it takes time for people to become accustomed to a new monetary system. The elderly will have a hard time switching from physical to digital cash. It's hoped that young people adopt digital payments for the full transformation of the world's economy to materialize sometime in the future. Just my thoughts