Another great question. I think you've nailed the answer already: building a central bank (CB) crypto would be for FOMO purposes. I don't even say that to mock! It's important for institutions to fiddle around with new technology, you never know what might be useful -- for instance, a blockchain-based system for interbank settlements could be a boring, nonsexy, but maybe significant application. The question to ask with crypto -- with any technology, frankly -- is: what can do I with this that I can't do another way? And what are the opportunity costs, as well as benefits, of using this new tech to accomplish that goal? And cryptos are a lot of downside and not a lot of upside from a CB perspective, for the reasons you spell out here.

BUT! I'm super interested in the development of CBDCs -- central bank digital currencies. There's a lot of really interesting things (from a policy perspective and an econ perspective) that you could do with well-designed digital cash that currently suck to do with existing mechanisms. A CBDC could be a big deal and do a lot of good -- it just wouldn't work the way (most) cryptos work today.