They don't need to reinvent the wheel
Today's money is already digital (apart from physical cash, apparently), which basically means they already have that "centralized database" in place and thus don't need to develop it from scratch. With that said, the Chinese central bank seems to have been developing their digital version of yuan from the ground up
Anyway, using someone else's design is not a good idea for a sovereign currency. Therefore, if a digital dollar is set to take off somewhere down the road, it will be their own design, that is to say, Fed's, not Ripple's or whoever else's. More or less, you can't outsource parts of your business which are vitally important for its long-term success
Good point. It makes sense to use a centralized database, instead of relying on an existent public blockchain network like XRP or Stellar. After all, governments want full control of the new monetary system. CBDCs will be considered as distributed ledgers, instead of Blockchains. That's largely because governments and central banks will have full control over the network's operations. My guess is that central banks will act as "miners", while governments will act as nodes of new digital fiat currencies (CBDCs). It'll be nothing more than a centralized database subject to a high-degree of surveillance in the mainstream world. In this scenario, physical bank branches will cease to exist. Banks will still be relevant in the new economy, but will provide their services entirely online.
Nonetheless, it seems to me that phasing out physical cash will take a lot longer than I've thought it would be. For instance, not every country has expressed their interest in launching a CBDC of their own. The United States is an good example of a country that's quite skeptical of launching a digital US Dollar even if China has already taken a step forward in this regard (with the development of a digital Yuan). After CBDCs become a reality, physical cash will still exist as some people use it for daily payments (especially the elderly). The process of phasing out physical cash will be a gradual one, as slowly but surely, CBDCs are introduced into every integral part of our society. Just my opinion