The properties of "unlikely to go down" don't require a trustless blockchain; the attributes of being resistant to a node being compromised don't require trust beyond consensus. git is unlikely to go down and verifiable. Copies of a git repo verified by gpg signatures and repos that only accept commits that pass verification requires only as much distribution as running three instances. The fed can do all of this without giving up any control.

There is a tremendous difference between "we have three servers so our stuff probably won't go down" and "the system could theoretically remain in use forever even if we disappeared from the world".

And that difference is gigawatts of power. A system that's "only" as reliable as Fedwire or SWIFT but uses 99% less power than cryptocurrency is an interesting tradeoff.