I'll cop to hand-waving and painting cryptocurrencies w/ a broad brush here. But it seems like Diem hasn't really figured out what its system is gonna be: https://en.wikipedia.org/wiki/Diem_(digital_currency)#Implem.... Looking at their docs and such, it seems like it'll be centralized under the stakeholders until they implement proof of stake, which IMO makes it just another reserve bank.
...no? Diem/Libra is built on Cosmos, which is a proof-of-stake system, not a proof-of-work system. Validator nodes in a proof-of-stake system do not compete to work ever harder to mine the same resources.
There are a rather large number of such validator nodes (100+ at minimum), but that's also true of banks when you translate their Highly-Available mainframe stacks into equivalent Highly-Available commodity-PC stacks. (I.e. they're not spending any more electricity per tx than the ACH system is.)